i want to play you something. tristan: you have to help her. please, please! (phone buzzes) someone has to help her. he's stabbing her! he's k... he's killing her! please, help! please! turn it off, now! we are gonna push through this. right now. he was attacking her. you rushed to help. he was exiting the apartment, he pulled the mask off. who was he? who killed isabel? (phone line ringing) damn it, avery, pick up. i can't get d.b. on his cell. (phone buzzing) i shouldn't have come here. i shouldn't have come here. i'm no help at all. this was a mistake. this was a mistake! i need to use the restroom. (line ringing) come on, raven, pick up. (phone buzzing) krumitz, why do you keep blowing up... raven, raven,

avery, tristan jenkins is the target. what? he was behind the wedial intrusion. he wouldn't say who was under the mask because it was him. come on. fbi! clear. fbi! clear! clear! this is special agent avery ryan. we have a suspect fleeing. tristan jenkins, five-foot-ten, caucasian, brown hair. i need him detained. right when you feel a cold sore, abreva can heal it in as few as two and a half days when used at the first sign. without it the virus spreads from cell to cell. only abreva penetrates deep and starts to work immediately to block the virus and protect healthy cells. you could heal your cold sore, fast,

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(siren wails, tires screech) man (over radio): car 1, go, go! fbi! go right! tristan jenkins! all clear! federal agents! (indistinct radio communications) it's all clear. (indistinct radio chatter) isabel's apartment. everything tristan told me

with isabel he learned by eavesdropping on her wedial video chat. the chinese dinner... all that happened while she was video chatting isabel: over the long weekend. it would be nice to spend a night in my old room. tristan: we liked the same chinese food. takeout tuesday. so weekends were movie marathons-- usually animated movies. ...skip the games and ask her out already. can't get the color of the ocean right. you'll look at the ocean... tristan: she loved painting. she introduced me to it. ...painting and you realize it's actually a million... this is like a cyber version of that movie rear window. but, so, they never actually met? the way he's profiling, no, not in the real world. tristan's ptsd masked more serious behavior: technology facilitative social isolation. he basically lived his whole life in this apartment. that explains all the eggs we found in the refrigerator. probably wasn't getting enough vitamin d from the sun. sounds like agoraphobia. no. agoraphobia is

a fear of being in a situation where you feel helpless or trapped. tristan's isolation was not born of fear, but of dependence. technology provided him with everything that he needed. he became addicted to experiencing the world online and over time, the only time he felt safe or comfortable was behind a computer screen. i bet the first time he saw isabel was when she moved into this apartment building. he used his wedial exploit to find her and then eavesdrop on her. what drove him to murder? primal urge. as his obsession grew, his self-imposed isolation. so he forced himself out his door, into her apartment, carved her up while she video-chatted with her parents. it's looking that way. and if so, tristan didn't collapse because he witnessed a murder. he was overwhelmed by sensory overload when he committed murder and left this apartment. (phone chimes)

all right, text from russell. " our killer's saliva is not in any law enforcement database." well, that makes sense. if he hasn't left this place in years, he hasn't had the opportunity to commit a previous crime. can you grab one of tristan jenkins' toothbrushes and get it to d.b. russell in the field office? look, a simple dna test should tell us if tristan's our killer. yeah, but with ten monitors? i mean, he's probably watching more than just isabel. we gotta find this guy quickly and put some cuffs on him. krumitz: just heard from raven. she's downloading tristan's computer. hey, man... you cool? yeah. you didn't eat the trout in the commissary, did you? no. um... let me ask you a question. if you were given opportunity to help francine before she got in trouble, would you have? i'd have done anything to help her. anything. yeah.

ramirez: avery? i think i got something here. through tristan's computer. i found online orders, message board posts, nothing special, but then i found this. are we live? no. these are recordings. eight terabytes worth, over 100 hours of video. i'm thinking tristan that came through his machine. wait a second. there's laughter, a birthday, a grandfather. these are all happy moments. so, tristan's doing what? collecting happy moments from his victims' lives? tristan: my mom and dad are still married. my sister goes to florida. she's a junior. oh, my brother just had a baby. grandma died a few years ago, but he loved her so much. he can't even replace the glasses grandma bought him, no matter how many times they break. there's a generational mix.

these people aren't his victims. they're his family. that's why i didn't read that tristan was lying. tristan so fully believes his delusion that he thinks he's telling the truth. ramirez: tristan thinks this collection of random strangers are his loved ones? to him, they aren't random strangers. they are his family. (phone buzzes) russell. okay, avery, i just got the dna results back from tristan's toothbrush. it's not a match to the saliva samples we got from the crime scene. he's not isabel's killer. all right, so tristan did not brave the outside world to kill isabel. but he knew that she was being murdered, because he was eavesdropping on her wedial session. he saw exactly what her parents saw. isabel: my job out here... ryan: he did everything he could to get her help without leaving his apartment. (dialing phone) man: 911. please state your emergency. he's stabbing her! please, help! help! and then when he realized the police were not gonna get there in time, he did the only thing he could. he overcame his fears and stepped outside to help. but if tristan's innocent, then,

ryan: you know, the last time we spoke, i think he might have recalled the killer's face. and now he's out for revenge. if you misplaced your discover card, you can now use freeze it to prevent new purchases on your account in seconds. and once you find it, you can switch it right on again. you're back! freeze it, only from discover.

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i've made up my mind. so i have to stop him. telling myself he was just overly affectionate, but it's sexual assault. i threatened to tell my father, but he wouldn't stop. so i told him i'm going to the police. ramirez: i found this video journal entry on isabel's computer. she deleted it, and it took some time to recover. there are no other videos like this. she doesn't identify who she's talking about, or where the assault took place. why would she record it and then delete it? shame. changed her mind.

this was motive. the " he" that she talks about... was scared. whoever this person is, is her killer. (murderer grunting, isabel screaming) (grunting and screaming continue) recovered this from tristan jenkins' hard drive. i had to dig through hours of footage. this is what isabel's mother and father witnessed. krumitz: unfortunately, this video help us i.d. our killer. okay, all right, uh, nelson, go back a few seconds, please. i really miss you guys. russell: right there. and highlight the engraving. it's the infinity symbol. what are you thinking? i'm thinking that we can run a facial recognition without a face. stands for automated identification, right? so we all have articles of clothing we wear so often, they become

unique identifiers, like our murderer's bracelet. well, this is great, but there's no random-stuff-we-own database to search. you know, actually there is, and that index happens to be public and linked directly to people's names. what is it? 350 million samples submitted every single day by the users themselves. friendagenda! pow! okay, crop out the unique characteristics of our killer. all right, then we write a script to crawl all friendagenda photos. and if isabel's murderer is like the rest of the world, he will that will lead us to him. 52 matches in the tampa area. with isabel stanworth? zilch. nelson: zero? the entire time i've worked here, i've never had okay, well... wait a minute. russell: here we go. all right, so, social media sites like friendagenda-- they-they link

krumitz: uh, yes, yeah. stay-stay with me. yeah, yeah. okay, when i analyzed the saliva samples that raven collected from the scene, the dna gave me, what? age, hair color, eye color, all of that stuff, but it also gave me evidence of an inheritable genetic disease. where is it? right here. okay, so, our killer has a 50% chance of inheriting marfan syndrome. okay, well, let's see if any of our 52 results have a parent that belongs to a support group on friendagenda. right. krumitz: got two hits. bringing up the profile now. the one on the right's our murderer. okay. what now? you could have given me the heads up, man. what are you talking about? talking about the fbi. they're raiding my firm right now. they won't even let us back in the building. you knew that this was gonna happen, brody, and you did nothing about it.

but i guess blood and family doesn't mean much of anything to you now?! look, that is not true, jordan. we're blood, but it wasn't me that decided we weren't family. (phone beeps off) (siren blaring) georgia: you're sure this is everything? i think so. but if i find anything else isabel had here, i'll mail it to you. she was my friend. get out! georgia: oh, my god! ple-please... please don't hurt us. gil, georgia, i didn't know you'd be here. do... do we know you? no, no, no, i'm not... i'm not gonna hurt you. i'm just here for him. he killed her. what? what are you talking about? i remember. i finally remember it. i saw your face when you took off the ski mask! neil, what the hell's he talking about? i don't know. she was my friend, and you took her from me.

no. is this true, neil? gil: look at me. georgia: oh, god. look at me, neil. oh, god, gil. gil: look at me! elevator's not working. tristan must have shut them down. he chained the stairwell, too, locked us out. there should be an emergency ladder inside the elevator shaft. it's not opening. get the hydraulic spreader. i can try to hack the elevator, get in electronically. no, we don't have time for that. we're gonna get to tristan the same way he got to isabel-- over the internet. to do this. yes, i do. for what you did. you took her from me. tristan, listen to me. want this for you. shut up! you don't know what she would have wanted! you didn't know her like i did! no one did. ryan: no, you're right. nobody knew her like you did. she shared all her secrets with you. of your family?

your grandpa. your brother with the new baby. your sister in college. tristan, if you do this, you will lose them all forever. put the gun down, tristan. put it down and keep living for your family. that is what isabel would want. (crying) hands in the air! hands in the air, on the ground now! get on the ground! you stay right there. (tristan whimpers, sobs) (sobbing): oh, my god. ryan: what's happening? (crying): oh, my god! gil! (georgia sobbing deeply, sobs echoing) mundo (echoing): take him away.

(indistinct radio transmission) wait... tristan. i didn't know gil was gonna be here. i was supposed to kill neil. me, not him. she's gone. she's gone, and i can never have her back. tristan... i don't even know if you can understand this, but you are gonna be brought up on charges, too. (siren whoops in distance) isabel would have liked you. captioning sponsored by cbs

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we begin with breaking news from worcester. >> weapons were stolen from the army reserve center over the weekend. the fbi is involved.>> reporter: sources have told us the weapons stolen our 10 pistols and six semi automatic wife rolls -- rifles. the fbi said this is not connected to terrorism. they want the weapons back. investigators worked late. these broken saturday night and stole weapons of. the fbi said there is no threat

to the new england area.>> we are not tying this to any specific threat. >> reporter: army personnel walk the ground.>> anytime you have terrorist groups or criminals breaking in to police stations or armories stealing weapons in order to threaten the local government or people come that is something we take seriously.>> reporter: investigators entered the stolen weapons into a national database. the governor addressed this. all levels of government are staying brave. he stressed the breaking would people will be chasing and investigating this.>> reporter: investigating.

the fbi said this case is not related to any others. a growing memorial in paris. people like landoll's -- light candles. france is in mourning tonight. they are making good on their promise to retaliate.>> a dozen bombs. police have made seven arrest. an international manhunt is underway. there is word from intelligence that there was a warning with eminent attack by isis one day before. >> reporter: there is a sense