like us on facebook and follow us on twitter. that's our program for tonight. don't go any where. "20/20" starts right now. tonight on "20/20" -- >> my name is kayla mueller. >> from this, to this. >> it's very terrifying here. >> a young american woman, devoted to helping others, kidnapped into a living hell by isis. her desperate parents, everyday people, forced to take on terrorists from their living room. because they say the u.s. government and various humanitarian groups failed them. >> they decided to leave our

daughter there. >> tonight, we're taking you inside the extraordinary story you've never seen before. a family never giving up. and now revealing a secret the world never knew. >> this is a plea for kayla's life. >> we're doing everything we can to get you home. >> the girl left behind. >> good evening. i'm elizabeth vargas. >> and i'm david muir. right here tonight, the stunning result of a two-year abc news investigation. parents who are revealing how they had to take on not only isis but the u.s. government to try to save their daughter. >> it's a story of inspiration, but also of two parents, outrage, and heartbreak. tonight, they're telling their story to brian ross, reporting on the girl left behind. >> reporter: a summe

prescott, arizona. >> monday morning at about 4:00 or 5:00 a.m. >> reporter: her friends had called to say their 25-year-old daughter kayla had gone missing. >> they called us and said, you know, they weren't really concerned. "don't worry." >> reporter: marsha and carl mueller soon learned from kayla herself that there was plenty to worry about. >> my name is kayla mueller. i need your help. it's very terrifying here. >> and then it went blank. >> reporter: kayla had been captured by one of the most brutal terror groups the world has ever known. isis. >> you just go into almost a catatonic state, i think. you just can't even stand up. >> reporter: carl owned an auto repair shop. marsha, a former nurse. nothing prepared the muellers for what was about to happen -- the scary, secret world they were about to enter. >> this kind of thing tears

>> reporter: as the world would later watch the brutal murders by isis of three young american men. >> i'm back, obama. >> reporter: the plight of a fourth american hostage, kayla, was kept a closely held secret. >> obviously the white house doesn't want this to all come out. >> reporter: tonight, on "20/20," the muellers are coming forward to tell the story of how they fought for kayla's freedom. >> kayla is not your enemy. >> reporter: the messages and e-mails with isis in syria and the muellers in arizona never seen or heard before tonight. >> sir, i am coming to you with a mother's heart for the love of her daughter. >> would your staff negotiate for kayla? >> reporter: and the people the muellers now say -- >> no. >> reporter: -- failed their daughter, including prominent humanitarian organizations, and the obama administration. >> the president could have been a hero and he chose not to be. >> hey, look! look at her. there's our little girl. >> what are you going to name

>> kayla? that's a pretty name. >> hi, kayla. we weren't supposed to have kayla. you know, we were told no children, and suddenly we were going to have this young little girl. get him, get him! she was a really happy little kid, and loved living. loved being in the world. >> she taught herself languages, she taught herself how to play the guitar, she taught herself how to write music. she was very intelligent, very intelligent. >> i hope i never forget that everything happens for a reason and that nothing should be a regret. >> she just always had such a compassion for everybody around her. >> kayla felt suffering in her heart and it was as if it just seemed natural she was supposed to help. >> reporter: after college, kayla was drawn to the middle east.

whose image caught the world's attention this month, there were thousands of children who kayla sought to help. >> and she met this syrian man who was there. >> reporter: his name was omar. and kayla joined him in the ancient town of antakya, or antioch, in turkey, about a half hour away from wartorn syria. amidst a local celebration at the antakya airport, we met up with two of the people who worked with kayla and became her close friends, a mother and daughter, orouba and halla barakat. >> to hear that there is an american who has left everything, and she came just to help out refugees and syrians in particular, that was very inspiring, honestly. >> many syrians live here. >> reporter: the barakats helped us to retrace kayla's steps, in the neighborhoods where she worked, with women and children whose husbands and fathers had been killed or left behind. and was she making a difference in the lives of people here? >> yeah.

just saying kayla's name in fr them, they smile. >> reporter: as long as she stayed here in turkey, kayla's parents were confident she was safe. safe, until omar asked her to go along on a quick trip across the border, into syria, to the city of aleppo, to help him install communications equipment for a hospital run by the humanitarian group doctors without borders. >> i told her, "no, you shouldn't go." >> reporter: you tried to talk her out of it? >> yes. i told her, "it's very dangerous." >> she was like, "don't worry. we have a lot of people with us. we'll just go, fix everything in the hospital and then come back." >> reporter: it was at a time when safety concerns were all about the cruelty of the syrian government, not isis. >> at that time isis was -- wasn't even heard of. and i said, "kayla, this is not your war and -- and these are not youreo

>> reporter: kayla and omar crossed into syria at this border checkpoint. at that time no other humanitarian aid worker had been kidnapped. it was august 3rd, 2013. and less than 24 hours after leaving here, kayla mueller would be plunged into what can only be described as a hell on earth. captured. imprisoned. tortured. raped. taken as a slave by the leader of isis. >> he was raping america. >> reporter: next, how this courageous young american from arizona remained strong in a place where others lost hope. >> know i am also fighting from my side and have a lot of fight left inside of me. >> reporter: as her parents faced the looming deadline to save her. >> every day to us is a critical time. every hour. >> reporter: kayla mueller was the girl left behind. basketball!

as an emergency umbrella. to help feed a friend. a drum solo! i just use my backpack for books and stuff. embark backpacks. guaranteed for one year. a good car has to maneuver quickly. that's also true of a good car company. people have always bought cars. but we saw an opportunity in sharing cars. so we moved fast and launched car2go in 29 cities, all around the world. doing that required dozens of data centers, designed for speed and performance. we built our business on the ibm cloud. because that's what the ibm cloud is built for. especially when it comes to on snacking.g new.

cheese dippers. the creamy cheese and crunchy breadsticks that go wherever you do. cheese dippers by the laughing cow. reinvent snacking. ♪ and one at the end of he the day. ♪ ♪ ♪ it's my belief that for instant relief... ♪ ♪ a hug is the best cure of all. ♪ what headache?in? what arthritis pain? advil makes pain a distant memory nothing works faster stronger or longer what pain? advil. the honey in honey nut cheerios

[sighs] that's why we make them for you. (cri♪ket chime) singing: cricket's got you covered like a blanket on a pig our plans cost little and our network is big. like a great, big gorilla in a pompadour wig great is great when big is big. you can text and like and swipe and stream it's the great, big network of your great, big dreams. it's the no duh uh-huh obvious callllllllllll it's the great, big network, sending love to us all.

"20/20" continues, with the girl left behind. >> my dear, dear, kayla. it has been two days and five and a half hours since we received the call that you were missing. >> reporter: every day that kayla was missing, her mother marsha would go into her daughter's empty bedroom and record the day's events. so when kayla came home, she would know what had happened. >> we're staying strong and

positive for you and we're asking god for a miracle and counting on it. >> my name is kayla mueller. >> reporter: but her parents saw the fear in her face -- >> i've been here too long, and i've been very sick. >> reporter: -- when isis sent this ten second video, as proof they were holding her. >> it's very terrifying here. >> reporter: no one yet knew, or could even imagine, just how brutal isis could be to its hostages. >> fear. it's fear of unknown. you don't know what is going to happen. >> reporter: as her fellow female hostages now describe it to "20/20," it looked something like this. isis was holding kayla in a 12-foot by 12-foot room, with a single light bulb hanging from the ceiling. >> there was a little bit of light coming by this small vent, but that was -- that was it. >> it was cold, dirty. >> reporter: kayla had been in captivity for six months when isis brought in three more female hostages. >> they gave us black dresses

and niqabs, to cover our heads and faces. >> reporter: and always in the background -- >> there was this music. the jihadi songs, and they played on and on and on. >> reporter: for the first time, tonight, two of kayla's former cellmates, frida saide and patricia chavez, are talking publicly about what happened to kayla and them at the hands of isis. >> we realized that they were actually killers, that they would enjoy killing us. >> reporter: we brought the two women together in stockholm, sweden, where frida now works. both women were staff members for doctors without borders, kidnapped after kayla. the prison they shared with kayla was located outside the isis headquarters city of raqqa, in what they thought was an oil refinery, in this building in the complex. kayla was happy to have company when they arrived. >> she'd been kept mostly in

>> reporter: but still holding out hope. >> she was amazing. she was a really strong girl. >> she had a strong faith that gave her a lot of strength. >> reporter: do you think kayla was treated any more harshly because she was an american? >> i think it was clear that they hated americans more than other nationalities. yes. they would scream at her, they would blame her for everything that america has done in the world. >> and i'm back because of your arrogant foreign policy towards the islamic state. >> reporter: their isis guards were led by the british recruit who would later be dubbed jihadi john as he carried out the beheadings and murders of at least ten hostages. the hostages called him and the three other british guards the beatles, but there was nothing fun about them. >> they caused so much pain to me and to others. >> reporter: sometimes taking the women to another room alone, shining a brig

face and demanding sensitive personal information, an emotional low point. >> it was important not to break in front of them, but then behind closed doors, yeah, we had our moments. >> reporter: at other times kayla was taken to the room next door where the male hostages were being held, paraded in front of them by jihadi john. hostage daniel rye, a danish freelance photographer, recalls how kayla stood up to the brutal isis guards. >> one of the beatles started to say, "oh, this is kayla, and she has been held all by herself. and she is much stronger than you guys. and she's much smarter. she -- she converted to islam." and then she was like, "no, i didn't." >> reporter: what? she said that? >> yeah. "i did not convert." >> reporter: daring to correct the violent jihadi john. that surprise you? >> it surprised me a lot.

i would not have had the guts to say that, i don't think so. >> reporter: and rye says kayla, and another american hostage, journalist steven sotloff, figured out a way to pass letters back and forth, creating a makeshift game of trivial pursuit. was it dangerous to be moving these letters back and forth? >> of course it was. >> reporter: but she did it. steven did it? >> yeah. if i've been by myself at her place, i would probably say, "no, no letters. it's too dangerous." >> reporter: back in arizona, kayla's parents held out hope that isis would treat an aid worker gently, that one of the humanitarian groups she was connected to would help negotiate her freedom. but that did not happen. >> no one would claim her, so they assumed she was a spy. and we found out that her fingernails had been pulled out. her hair had been shaved and that she had been tortured. >> beaten.

>> and that was just one of the hardest days for me. >> reporter: and the prospect of a negotiated release grew increasingly unlikely as the muellers soon found themselves up against the u.s. policy against making concessions to terrorists. >> i firmly believe that the united states government paying ransom to terrorists risks endangering more americans and funding the very terrorism that we're trying to stop. and so i firmly believe that our policy ultimately puts fewer americans at risk. >> reporter: isis was making millions of dollars from its kidnapping business. and as hostages from other countries which allowed ransom payments were released by isis, they could only say good-bye as they left kayla behind. >> it was a horrible feeling to be released, looking forward to being released but at the same time leaving someone behind. >> reporter: doctors without borders negotiated ransom

payments for its staff members, frida, patricia and three others. but not kayla. >> it was very sad. i think it was the saddest moment we passed through together. >> reporter: how did you say good-bye? >> with tears. >> reporter: what did you say to her? >> "stay strong. this will end one day." >> reporter: and what did she say to you? >> nothing. >> reporter: but as they left, a possible break. the isis hostage takers told the women they were now ready to negotiate for kayla. and a remarkable e-mail back and forth was about to begin between the terrorists holding kayla, and her desperate parents in arizona.

realpack uber flavor into ultra thin. sargento ultra thin slices with just 45 calories a slice- that's 45 reasons to layer, stack sneak, peek, beg, like, love. sargento- we're real cheese people. they're all gone. i know i bought them. well, staples has low prices. if i were you, i'd grab a couple more... for next week. back to school or back for more? staples has the lowest prices... period. staples. make more happen. staples has the lowest prices... period. introducing t-mobile's most epic deal ever! get a free samsung galaxy for everyone in the family. that's right, a free samsung galaxy with every new line and get 4 lines with 6 gigs each for just 30 bucks a line.

heir favorite services. don't wait. get a free samsung galaxy for everyone. so get t-mobile now. because our most epic deal ever is only for limited time. i absolutely love my new but the rent is outrageous. good thing geico offers affordable renters insurance. with great coverage it protects my personal belongings should they get damaged, stolen or destroyed. [doorbell] uh, excuse me. delivery. hey. lo mein, szechwan chicken, chopsticks, soy sauce and you got some fortune cookies. have a good one. ah, these small new york apartments... protect your belongings. let geico help you with renters insurance.

with this level of engineering... it's a performance machine. with this degree of intelligence... it's a supercomputer. with this grade of protection... it's a fortress. and with this standard of luxury... it's an oasis. introducing the completely redesigned e-class. it's everything you need it to be... and more. lease the e300 for $549 a month at your local mercedes-benz dealer. mercedes-benz. the best or nothing.

everyone, if you are receiving this, it means i am still detained. >> reporter: the voice isn't kayla's,

coming from the secret isis prison where she was held in the syrian desert. >> please know that i am in a safe location, completely unharmed and healthy, put on weight in fact. >> reporter: after eight months of silence, kayla's words now appeared in a letter in her handwriting, smuggled out by the women from doctors without borders, who had been released. >> know i am also fighting from my side in the ways i am able, and have a lot of fight left inside of me. >> not only was she alive, she was strong still. >> do not fear for me and continue to pray. all my everything, kayla. >> reporter: but with no way to contact the kidnappers, the muellers could only wait for the next step, unaware that officials at doctors without borders at their headquarters in brussels had purposely withheld for almost two months that very contact information brought out by kayla's fellow hostages.

>> the women were given this e-mail to negotiate for kayla and didn't even give it to us, much less do anything to help her or help us. >> we could not share it with you because of our safety concerns for o staff. >> reporter: the group's director for syria said she had waited for all of her staff to be free, in a phone conversation recorded by the tearful muellers. >> would your staff negotiate for kayla? >> no, because it will be difficult for us to negotiate for kayla. the crisis management team that we have installed for our five people will be closed down in the next week. because our case is closed. >> they left her there. they deserted her. they were instructed by the hostage takers, "negotiate for this woman." >> reporter: and they did not. >> they did nothing and wouldn't help us.

you'll send that e-mail right away after we hang up? >> we prefer to send it tomorrow if that's okay? >> well, not really. >> we'd like to get it to our contact as soon as possible. >> every day to us is a critical time. every hour. >> okay. yes. okay. yes. >> at least we got an e-mail. >> reporter: few humanitarian groups in the world are more highly praised than doctors without borders, known overseas as msf, the letters of its name in french, medecins sans frontiers. >> msf operates in places where there are wars, natural disasters. >> they're a fabulous organization, and they do wonderful work. but somewhere in a boardroom,

they decided to leave our daughter there to be tortured and raped. >> reporter: kayla was in a doctors without borders vehicle when she was captured by isis in syria, but the organization said nonstaff members are not its responsibility. >> we could not take steps to negotiate for her freedom. >> reporter: why not? >> because we can't be in the business of negotiating for people who don't work for us. and to do so would also have increased the risk that our teams already face. >> reporter: the executive director of doctors without borders usa, jason cone, says the group never would have allowed an american to come to its hospital in syria if it had known of kayla's plans in advance. >> reporter: but once she was taken, wasn't she your responsibility? >> no, she wasn't our responsibility anymore. >> reporter: was there no moral responsibility? >> i don't think there was a moral responsibility. >> i don't understand how a humanitarian organization can justify doing that. >> reporter: finally, in late may, 2014, it was isis which made contact with the muellers in this e-mail, likely written

by one of the british isis guards. >> this message is to inform you that we have the american citizen, kayla jean mueller, prisoner. we don't want to harm her. she's like a guest with us at the moment. >> they're telling you they're open to negotiation. >> reporter: chris voss, who once ran the fbi's international hostage negotiation team, says the e-mail and the earlier video were clear signs that a deal could get done with isis. >> this is, "we have something that you want. we want you to believe you can get her out because we want something for her." >> reporter: even though she's an american and america says it doesn't negotiate? >> right. because they know that if -- if given the opportunity, families will find a way. >> the conditions of kayla's returning home safely are no media involvement whatsoever, and a cash payment. >> kayla's a commodity unfortunately. they make an opening offer. they want some sort of counteroffer to continue the conversation.

sign -- isis would allow kayla to answer three personal questions to prove she was alive. carl wrote back -- >> how did you get stitches in your eyebrow as a child? >> her older brother eric was pulling her in a wagon and it tipped over. >> reporter: correct. >> what did you teach your niece to say, music is -- >> music is everywhere. >> reporter: correct. >> what is your friend moe's real name? >> her name is monica. >> reporter: all correct. it was kayla. >> mom and dad, i'm still remaining healthy. you should have already received the three answers to the proof of life questions you provided. >> reporter: even better than the e-mails, a followup with kayla's actual voice, laying out the kidnappers' latest demands, including the release of a pakistani woman being held in a u.s. prison. >> those detaining me are demanding an exchange of dr. aafia siddiqui's release for my release. if this is not achievable, they are demanding five million euros to ense

good-bye. >> they're saying she can still be released. >> reporter: working with kayla's former campus minister, reverend kathleen day, the muellers began what would be a remarkable exchange of some 27 e-mails between the arizona couple in their kitchen and the terrorists holding their daughter in syria. >> they were focused every day, every waking moment, every sleeping moment, on what they could do to get kayla back. >> reporter: the muellers' answers were all crafted by fbi agents assigned to the case from the very beginning. and reverend day began to be worried by what other aid groups had told her about working with the u.s. government in hostage situations. what was the concern? >> that if you worked with the state department that you'd never get your people out. >> reporter: and soon the muellers began to have their own doubts. you asked to meet with president obama? >> he refused. we did meet in the white house with many high-ranking officials but he did not see us.

>> reporter: and no one in the white house told them what was about to happen. u.s. special forces, which have carried out a number of successful high-risk rescue missions, like this one, had been authorized to launch a secret rescue operation for kayla and some 18 other hostages. >> it turned out that the hostages were no longer at that location. >> reporter: it was a dry hole. they just missed them. >> i deployed an entire operation at significant risk to rescue not only her, but the other individuals that had been held, and probably missed them by a day or two. >> reporter: one week later, in arizona. >> we got a nasty e-mail from isis that said because of what your arrogant government attempted to do, your daughter has 30 days to live. >> if you fail to meet this deadline we will send you a picture of kayla's dead body.

>> it was kayla's birthday that was the deadline for them to kill her. >> this will be our first act of revenge taken for the miserably failed and unsuccessful attempt by your arrogant government to free their prisoners. >> reporter: now the muellers had just 30 days to raise the money and get it to isis, and even harder, overcome the u.s. government efforts to stop them. >> it's a family's darkest hour. >> reporter: kayla's life hung in the balance. get your jeans game on at kohl's. get to kohl's nowts! for great savings on all the denim your kids want! get every shade of blue and even polk-a-dots too! plus yes2you rewards members earn 1 point for every dollar spent. get a $5 reward for every 100 points! now that's the good stuff. kohl's.

gto discover the power of the human hand. the ability of a lexus master craftsman to turn an ordinary experience into an extraordinary one. ♪ for a limited time get some of the best offers of the year on our most refined vehicles at the lexus golden opportunity sales event. get up to $5,000 customer cash on select 2016 models. ends september 5th. see your lexus dealer. americans, i don't understand you. always working on vacation. always multi-tasking. always busy making something. but what has all this ambition ever gotten you? that you're the greatest nation in the world? where all your dreams come true? the galaxy note7 with s pen. perfect for busy americans like us. [bark] do more, with the new galaxy note7.

luann bennett. after losing her husband to cancer, she raised three boys here in northern virginia and grew the family business. a single working mom who helped create over 1,000 local jobs, bringing people together to solve problems. i'm luann bennett.

one to the table and work to get results. congress just doesn't get that-- there's too much partisanship. i approve this message because washington needs more common-sense problem solvers. >> reporter: it was two years ago this summer that americans were hearing about isis for the first time, as its leader abu bakr al baghdadi emerged from the shadows, presenting a threat that was now greater than al qaeda. but there was no mention in the news of the american hostages, kayla mueller, james foley, steven sotloff and peter kassig. their plight remained a closely guarded secret.

>> my name is carl mueller, this is my wife marsha. >> reporter: as the 30-day deadline for paying a ransom approached, the muellers recorded this dramatic personal plea, scripted largely by the fbi, and sent to the new isis leader al baghdadi. >> this is a plea for kayla's life. we are asking for your mercy for kayla, as we believe you are the only one who can grant us this. >> these are two people that are in the midst of a horrific experience. >> i beg you not to harm our daughter. >> they can't imagine that they would ever be trying to talk on a camera in a respectful fashion to someone who likely going to murder their daughter. >> our hands have been tied in trying to raise money, yet we continue to do everything in our power to work toward securing kayla's freedom. >> sir, i am coming to you with a mother's heart, for the love of her daughter. >> her mother's got her hair covered. that's critical. it shows respect.

your power to free our daughter. >> that's the way you buy time. that's the way you keep somebody alive one more day. >> reporter: it seemed to have worked. the deadline for kayla's execution, on her birthday, august 14th, passed with no word of her fate, a sigh of relief and a hope that she had seen this part of the video, too. >> kayla, we love you. we love who you are, and we admire your compassionate heart and devotion to helping those that suffer. we know god is comforting you and strengthening you. i wish we could see you. >> reporter: but it was a very short-lived moment of relief. >> president obama is about to speak from martha's vineyard on the execution of american journalist james foley by the islamic militants fighting in iraq and syria. >> reporter: the first of kayla's fellow american hostages to be murdered. >> today the entire word is appalled by the brutal murder of jim foley by the terrorist group isil. >> reporter: again the muellers ask to meet with prede

>> we asked to see him and we were told no. >> reporter: a few weeks later, another american hostage killed, journalist steven sotloff, the hostage with whom kayla had grown close in captivity, with their makeshift trivial pursuit game. >> overnight our government determined that, tragically, stephen was taken from us in a horrific act of violence. >> reporter: and now the president agreed to meet with the muellers and the family of the other remaining american hostage, peter kassig. >> the president was very, i felt, cold. he said, i'm a father, i have two daughters. if that was my daughter being held, i'd do everything i could to get her out. and i'm sure he would have. but we were told many, many times, "we're doing everything we can, everything we can." and they weren't. >> reporter: they were not? >> well, no, they weren't. >> reporter: what the muellers

wanted the president to do was allow them to just make an offer of ransom money to isis for kayla's life. >> carl would say we need to make an offer and then the e-mail would not have anything about an offer in it. >> reporter: because they were all scripted by the fbi? >> right. >> we were like sheep. we were following what the government told us to do. we had no idea. >> reporter: in one e-mail, sent under carl's name, a suggestion of financial hardship. kayla may not know that i retired this january. isis was unmoved. >> retiring will not help you get your daughter back, so go back to work and earn some money! >> reporter: isis was running out of patience. >> so no more sentimental sob stories! >> reporter: and carl was increasingly unhappy with what the fbi had written for him to send to isis. >> at one point i even said to the team, i said if i got this e-mail back i would be really mad. >> reporter: isis stopped responding. >> after a while they got the message that this was only stalling.

voss says the fbi should have let the muellers make some kind of a counteroffer. >> the kidnappers are begging for anything to be thrown onto the table. anything. at one point in time, they said, "just give us a benchmark." >> reporter: should there have been a counteroffer? >> yes. keep the conversation going. keep the hostage alive. buy more time for a tactical resolution. >> reporter: but that did not happen? >> as far as i know they were never allowed. they were told if they made any sort of an offer they'd be prosecuted, which is unconscionable. >> reporter: a white house official, part of its national security staff, delivered that threat of prosecution to the muellers and all the other american hostage families, including the parents of james foley, who went public after he was killed. >> we were told very clearly three times that it was illegal for us to try to ransom our son out and that we had the possibility of being prosecuted. and yet meanwhile our son was being tortured and beaten every day. >> reporter: but kayla was still

alive and the muellers had one more option, a last-ditch trip to the middle east where one government said it was prepared to negotiate for kayla's freedom. as she was about to come face to face with the leader of isis. co, but i switched to sprint. their network reliability is now within 1% of verizon. and, get this, with sprint's new unlimited plan, you have the freedom to do virtually whatever you want. is there no worrying about gigabytes, shmigabytes, overages, bill phobia? that's the dream right there. well, then mama is gonna text, ping, post, tweet, snap... those dogs that learned to dance on their hind legs... can you hear that? don't let a 1% difference limit your freedom. unlimited mobile optimized data. four lines $160 a month. live your life unlimited. for people with hearing loss, visit sprintrelay.com.

we were learning about how talented the ancient greeks were, and suddenly i traveled back in time! i thought, "i could have been a writer." or an athlete! i could have even been an architect! then i realized, i don't have to go back in time to do amazing things. i can start today. yes i can. tag, you're it. tag! you're it. tag, you're it. yes! new gogurt write-on tubes. this back to school, say it with gogurt.

but how did we end up here? his mom thought he had the flu and that he was covered by the meningococcal meningitis vaccine he had received. until 2014 there were no vaccines for meningitis b in the u.s. now there are.

lead to death within 24 hours. trumenba is a vaccine for 10 through 25 year olds to help prevent group b meningococcal disease. trumenba should not be given if you had a severe allergic reaction after a previous dose. most common side effects were injection-site pain, fatigue... headache, muscle pain, and chills. ask your doctor or pharmacist about all the risks and benefits of trumenba and tell them if you've received any other meningitis b vaccines. meningitis b can be spread by typical sharing like... a drink... a spoon... a kiss. it all started here... it might have been prevented with trumenba. ask your doctor or pharmacist about trumenba.

"20/20" continues with the girl left behind. >> 'twas the night before christmas and all through the house. >> there's kayla! wanting her christmas. >> hi, kayla! >> kayla is ready. >> there's the christmas tree. and there's my mom on christmas eve. >> reporter: with kayla missing now for two christmases in a row, the family's only joy came in the letters she had written in captivity. >> i have had many hours to think how only in your absence

have i come to realize the gift that is each one of you. >> this is peter edward kassig. >> reporter: by then, isis had killed the third american hostage, aid worker peter kassig. now the only american held by isis was kayla. >> i had come to a point where our government is not going to bring kayla home. and she was still putting all her trust in the government. this kind of thing tears people apart. i don't know how we survived but i remember the argument. "marsha, we need to do something. they are not going to bring kayla home." >> reporter: the military option to rescue kayla had failed. the white house had threatened prosecution if they paid ransom. and the e-mails from isis had stopped. nothing had worked. >> i know that there are some very talented hostage negotiators in the fbi that knew

what they were doing. i see no evidence of their voices in these e-mails. >> reporter: for example, says former fbi chief hostage negotiator chris voss, his former colleagues should have picked up on the use in the very first isis e-mail of the word guest, which has special meaning in the arab world. >> you have to protectnd defend your guests with your life. this is something huge that they missed because they should have responded with, "no, she's not like a guest. she is a guest and she is your responsibility as a guest." >> reporter: and voss says the muellers should have been told there was a legal way to pay ransom if the ransom money might help track down the terrorists. >> it's been done a number of times with the department of justice understanding. >> reporter: so in this case, the u.s. could have allowed the payment of ransom to isis for kayla under the exception that it would help track them down and be a lure or a bait? >> yes. absolutely. >> reporter: so they missed a huge opportunity? >> i think they did. >> reporter: but the muellers had one last hope, a government

had served as a go-between with terror groups in gaining the freedom of other western hostages. >> we went over there to the middle east and saw several high-ranking officials. and the last one we met with came into the room, and he said, "and by the way, i just talked to john kerry and we don't pay ransom." >> then he went on to say, "i don't understand you americans. we believe if you can save one soul, it's worth trying." >> reporter: yet, the u.s. government had just used qatar to broker a deal for the return of american soldier bowe bergdahl, trading five taliban prisoners for bergdahl's freedom. the muellers say they can't understand why the u.s. would not again use qatar to make a deal for kayla's freedom. you must have been crushed. >> yeah.

it was one of our last gasps. we try to save our daughter and our government slammed that door. >> we will never stop working to set our citizens free. and bring them home to their families. >> reporter: a spokesman for secretary of state kerry said he had worked hard personally to try to gain freedom for kayla. but as dark and as hopeless as it seemed to the muellers, they did receive one piece of what was described by the fbi as good news. >> they told us she had been actually taken out of the prison by abu bakr al baghdadi, the caliph of isis. they believed she was with a family, that she was safe. so my thought was, "oh, good, kayla's able to maybe take care of the children. she's a maid." >> reporter: nothing could have been further from the truth, as the muellers later discovered. the so-called family where kayla was safe, was, in fact, holding her for the leader of is

>> one day kayla was taken and she came back and she went under her cover, under her blanket, and she just cried, just laid there and cried. and that's when they said, "he married her." that's the way they put it, their way of saying he raped her. so -- >> reporter: but baghdadi chose her? >> but look at the symbolism in that. he's raping america. >> reporter: hostages with kayla at the time say she remained defiant, and that even when she had the chance to escape, kayla put others first. this then 13-year-old girl, a member of the yazidi tribe who wanted to be called julia, was also being held with kayla as an isis slave. >> translator: i told kayla we want to escape and i asked her to come with us. she told me no, because i am american, if i escape with you, they will do everything to find us again. it is better for you to escape alone. i will stay here.

>> reporter: so she stayed behind? >> translator: yes, yes. >> reporter: under a full moon, julia and another yazidi girl made their way to freedom, tearfully leaving kayla behind. >> translator: i will never forget her sacrifice. i will never forget. >> reporter: four months later, in prescott, arizona. >> my dear beautiful kayla. i miss you and love you. yesterday, friday, at about yesterday, friday, at about 10:00 a.m., we were called -- >> good evening and we begin tonight with the breaking headline. >> he told us it was out on twitter.

>> tonight isis says that woman, kayla mueller, 26 years old, is dead. >> my eyes are clouding up, sorry. still not believing the horrific people had lied so much and were so evil. >> reporter: precisely how kayla was killed remained a mystery. the muellers refused to believe that kayla was dead, until isis a few days later, sent three photos of kayla's body. >> i just kept still writing to kayla. hey, sweetheart, have so much to share. you, young lady, have taught me so very much. i will see you soon. love, mom. ♪ the sun'll come out tomorrow... ♪

tomorrow is not a given. but entresto is a medicine that helps make more morrows possible. tomorrow, tomorrow... ♪ ♪ i love ya, tomorrow in the largest heart failure study ever. entresto helped more people stay alive and out of the hospital than a leading heart failure medicine. women who are pregnant must not take entresto. it can cause harm or death an unborn baby. don't take entresto with an ace inhibitor or aliskiren. if you've had angioedema while taking an ace or arb medicine, don't take entresto. the most serious side effects are angioedema, low blood pressure... ...kidney problems, or high potassium in your blood. ♪ tomorrow, tomorrow i love ya, tomorrow.♪ ask your heart doctor about entresto. and help make tomorrow possible. ♪ you're only a day away ♪

especially when it comes to snacking.g new. that's why we're introducing cheese dippers. the creamy cheese and crunchy breadsticks that go wherever you do. cheese dippers by the laughing cow. reinvent snacking. sleep number beds with sleepiq technology give you the knowledge to adjust for the best sleep ever. the time is now for the biggest sale of the year, where all beds are on sale! save 50% on the labor day limited edition bed. only at a sleep number store.

olive garden's buy onstarting at $12.99. because there's nothing like a great meal with the family. except maybe another great meal with the family. buy one take one. choose one delicious entrée, like our new pepperoni fettuccine alfredo or our chicken pot pie italiano. plus unlimited salad and breadsticks at our place, and another for yours, for free. starting at $12.99. may all your tomorrows be as delicious as today. olive garden.

[ cellphone vibrates ] [ cellphone vibrates ] [ cellphone vibrates ] [ cellphone vibrates ] [ bell rings ] buy online. ready in an hour. weren't you just...? got it. staples. make more happen. my immediate reaction is heartbreak. she was an outstanding young woman and a great spirit. and i think that spirit will live on. >> reporter: you did meet with the president? >> just the three of us. he was sincere and apologetic and "so sorry" and this, that, the other. >> he asked what he could do for you. >> and i said, mr. president, i'd be remiss if i didn't give

you the opportunity to support kayla's foundation. and he said, "i will help that foundation." he says, "you won't know, it'll be an anonymous donation but i will." i'm still waiting. >> reporter: there hasn't been such a donation? >> no. i'm still waiting for that donation, mr. president. and you see the flag at my driveway. it's not something i take lightly, talking about our president that way. but that's what he said. >> reporter: the white house confirmed to "20/20" that the president has indeed not yet made the donation he promised 18 months ago. for the muellers, the foundation they created, kayla's hands, is so important and the president's failure to support it so disappointing, because it keeps her memory and legacy alive.

>> she just had a heart to help children, make children happy, make them laugh. >> reporter: this prescott, arizona, playground funded by the kiwanis club in kayla's memory will be dedicated in a ceremony tomorrow. >> kayla was one courageous voice, calling the world to speak with one voice and with our actions, to have the same courage and commitment to give all our everything for the sake of peace, justice and religious freedom. thank you. [ applause ] >> so what i have to say to all my loved ones in prescott is that i will always love you and it's never good-bye among friends. and that you're always learning

learning about life and that process never stops. >> the muellers -- the white house now says it will allow american families to privately pay ransom without prosecution. and the story of another young woman determined to help children. you can see it at abcnews.com. i'm david muir. >> and i'm elizabeth vargas. from all of us here at "20/20" and abc news, have a good night, leon: new zika concerns.

leon: we begin tonight with breaking news, the composite sketch of a suspect and attempted sexual assaults on the w&od trail. take a close look at this sketch. this is one of the suspects. the police believe this is not related to a second