this is "nightline." >> jimmy: tonight, he was charged with murdering his co-worker even though someone else's blood was found at the scene. tonight how a powerhouse attorney helped get this innocent man's conviction overturned and we are right there when he steps out of prison a free man. >> how does it feel to be free? plus, overnight sensation. ♪ got a feeling that i'm going under ♪ >> the fame came quickly for sean mendez. after his justin bieber-inspired vine video went viral. how he went from a six-second clip -- ♪ how we needed stitches >> to touring with taylor swift. but first the "nightline" 5. >> when pain goes deep, i use salonpas deep relieving gel.

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good evening and we start here tonight with an incredible reversal of fortune for one man sentenced to nearly three decades in jail for murdering his co-worker. abc's ryan smith has been following this case for years and when a bombshell development changed everything, our cameras were right there as an innocent man walked free.

>> reporter: the first time i met mario kasharo was inside menard correctional center, one of the toughest maximum security prisons in the country. >> can you make it 26 years in here? >> i don't want to. i don't want to sit every day thinking about how did i get here? >> reporter: the college graduate with a bright future a convicted murderer. shackled and chained to the floor. mario was locked up for killing 17-year-old brian care rick, wrongfully convicted, he said. >> are you responsible in any way for his death? >> no. >> reporter: his family determined to prove his innocence. >> there was no physical evidence, no dna. >> reporter: hired powerhouse attorney kathleen zellner. >> the prosecutors overreached extraordinarily to get this conviction. >> reporter: kathleen said the state was dead wrong from the start. and in reviewing the evidence readily available she became convinced she knew the identity of the real killer and was determined to set mario free. >> this will just be reversed outright. >> reporter: it's part of a

twisted murder mystery that's haunted the small village of johnsburg, illinois, for more than 12 years. winter 2002, 19-year-old mario was working at his father's grocery store, val's foods. five days before christmas one of the stock boys, brian care rick, walked into the store looking for a co-worker and was never seen again. >> holy mary, mother of god -- >> reporter: his body never found. his mother terry heartbroken. >> the torture of not knowing where he is, every parent's worst nightmare. >> reporter: authorities found traces of blood in and around the produce cooler. suspecting foul play, they questioned everyone. including three stock boys. shane lam, ron render, and mario kasharo, but quickly turned their attention to the boss' son, mario. >> completely shocked. >> mario's one of the nicest -- just a sweetheart. >> reporter: cops were beginning

to develop a more sinister profile as brian's father william kerik would share with the local reporter. >> mario allegedly was selling dope. and i think he cowearsed my son into working for him. >> reporter: mario admitted to sometimes selling pot. but denied ever running a drug ring. >> i would i guess sell people pot out of my personal stash. it wasn't anything that i ever -- it wasn't a criminal enterprise. >> reporter: authorities were convinced that brian was killed over a $400 or $500 drug debt. but with no eyewitnesses or a body, the case went cold. over the next eight years, mario would graduate from college. take the reins of the family business. brian's disappearance a distant and painful memory. until an eyewitness came forward -- >> hit brian a few times. >> reporter: with a tale of violence from the produce cooler. >> bleeding from his mouth, fell out. >> how hard? >> knocked him out. >> reporter: shane lam, one of

the stock boys. he previously denied knowing anything about brian's disappearance but now that he was in jail facing up to 12 years on drug charges he had a story to tell. in exchange for a deal and full immunity on the crime against brian shane recalled how mario wanted him to help settle a debt. >> went over there. told brian, what's up with the money you owe him? pay him money back. we got to arguing. mario said it was getting too loud. go in the produce cooler. >> reporter: that's when shane said things turned violent. he said he knocked brian unconscious and then mario told him to leave the cooler. >> he said, go away, i'll handle this. i left, went back to the party. >> reporter: in court prosecutors downplayed the blood evidence and focused on mario's role as the mastermind to shane's muscle. though shane says mario never laid a hand on brian, prosecutors insipsed he was guilty of the rare charm of murder with intimidation. by uttering the words "talk to him" they say mario was responsible for everything that

followed, including brian's murder. the strategy worked and mario was sentenced to 26 years. his family determined to set him free. >> we're going to fight to the very end because we're 100% behind him. >> reporter: attorney kathleen zellner says you only have to look at the crime scene to know the real killer is not mario or shane. >> we know that rob render and brian carrick had an altercation because both their blood is at the crime scene. >> reporter: brian's blood in the hallway, rob's bloody fingerprint on the cooler door handle and inside the door more of rob render's blood. evidence kathleen says the state all but ignored at trial. when police asked rob why so much of his blood was found near the cooler -- >> cut my finger, who knows. maybe i bit my nail so bad that it bled a little bit. >> there's no way that this amount of blood could have been left by rob render biting his nails. >> reporter: for kathleen, there

was motive. brian turned in rob for stealing booze, demanded monoed him for pot, and a new witness. >> what did this new witness say? >> said that remembereder had made a statement to him just a week before brian disappeared that he was very angry with brian and that he was going to jump him. with a weapon. >> reporter: that matched her version of events. kathleen said the fatal blow wasn't from a punch, as shane lam said, but from a knife. >> i believe that render comed up behind him like this and cuts his throat. >> reporter: render was never called to testify. after years of struggling with drug addiction, he overdosed in 2011. mindy lindholm, rob's older sister, passionately defends her brother. >> the premise that my brother could have possibly killed brian over $30 or over him telling on him or anything as ridiculous, my brother having an explosive temper is absolutely ridiculous.

>> i'm sure prosecutors will say this is a classic case of blaming the dead guy. >> doesn't mean dead guys didn't commit murders. the only person that owes money to brian carrick is render. the only person who's ever described wanting to jump him with a weapon is render. the only person in that back hallway is render. >> reporter: another bombshell. the state's star witness, shane lam, whose testimony put mario behind bars -- made a shocking about-face. >> all of it was false. every single thing. the state set it up. they said i'd be indicted for murder if i didn't cooperate. >> reporter: in a statement to abc news, the prosecutor's office denied coaching shane lam out of the presence of his lawyer, calling it unworthy of belief, untrue, and far-fetched. but shane's recantatation combined with confess evidence against mario amounted to a powerful appeal of mario's murder conviction. last week the appellate court of

illinois reversed the decision. a panel of judges slammed the prosecution's case, calling their theory unreasonable, improbable, and unsatisfactory. two years into his 26-year sentence -- mario walked out of prison a free man. greeted by a busload of cheering family members and friends. >> how does it feel to be free? >> amazing. it's an amazing feeling. >> reporter: the prosecutors can appeal but mario's attorney kathleen says their case is air tight. >> he will never spend another day behind bars. never. this thing's over. i think the decision clearly says that they helped fabricate evidence against mario. >> reporter: we reached out to the prosecutor's office but they declined to comment. newly freed, mario says he's grateful to his family and his legal team and shares his new dream. >> a one, a three, a five-year

plan, maybe. one would be, to be in law school a year from now. three, to be finished. five, to be a state legislator. >> reporter: hoping to right the wrongs others may face. >> i want to try and prevent this from happening again. and again and again. >> reporter: for "nightline," ryan smith, abc news, new york. next, it started with justin bieber-inspired videos on social media. now he's actually on tour with taylor swift. sean mendes tells us about stitching together an unusual path to fame. rheumatoid arthritis like me... and you're talking to a rheumatologist about a biologic, this is humira. this is humira helping to relieve my pain and protect my joints from further damage. this is humira helping me reach for more. doctors have been prescribing humira for more than 10 years.

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when a young man named sean mendes first started posting videos of his justin bieber cover songs on the internet, he never expected that one of them would go viral and that he'd soon become a pop star in his own right. tonight mendes lets abc's chris connelly into the whirl wind his life has now become. >> reporter: the seen screams of

young female pop music fans may be familiar -- >> i hope you enjoy the show, guys. >> reporter: the face and the name that they're screaming for may not. ♪ i've been hurt before >> reporter: this is sean mendes, a 17-year-old singer/songwriter, on his own onstage with just his voice, his guitar, and that smile. this canadian can draw a crowd of thousands in los angeles. ♪ you watch me >> reporter: watching him play such wide-eyed hits as "stitches." >> when you're out there and your fans sing it back to you what goes through your heart? >> it's the best. there's feeling like having your fans singing your song back to you. >> he's really hot, he's really hot. >> i'm sorry it took so long. >> reporter: it's the climax to a full day which began hours earlier. ♪ >> reporter: on the go sean

serenading a smaller group, the fortunate ones at his vip preshow. >> to see him in person is just like -- the best. >> i was trying to get my phone to take a picture and i couldn't do it. >> he looks like an angel came from above. he smiles at you and it's like -- >> not even breathless adoration can stop the clock. >> i love him so much! >> it's 4:54, we have to be out of here by 5:30. >> reporter: sean is due at the teen choice awards across town. >> sean mendes! >> reporter: at the show he wins a tca for music web star. >> for the fans, i live you so much, thank you so much. >>dy sound as nervous as i was? i hate speaking. i suck at speaking. stick to guitar. today is nuts. just won a teen choice award, now going to play at the greek? pretty cool. >> reporter: performing with big beats, dance moves, or high-tech production, mendes is refreshingly old school.

yet his rise has everything to do with social media. and its ability to mint new music stars at warp speed. >> could you have done it without a vine, a facebook? >> to be honest i don't think i would have had the confidence to step in the street and start singing. >> reporter: two years ago sean mendes had never sung a pop song outside his home in pickering, ontario. he'd grown fascinated by hits on youtube, the same diy efforts that got justin bieber noticed. >> so intrigued with the fact that someone could take a popular song and make it their own. and that on top of that, people could like that more than the original. >> when you start thinking, that could be me? i could try that? >> i had one friend and she was like, i know you sing, why don't you try it, post it? >> reporter: which he did. not only on youtube, as bieber had, but on facebook and on vine. >> it's just so unique at the time. you know, no one was doing that.

i didn't do it to become famous, i didn't do it to get recognition. i just did it for fun. >> reporter: until one day in august 2013, when he posted six seconds of himself singing bieber's hit "as long as you love me." it was an overnight smash. >> was that frightening when it blew up? >> totally. like, i don't want that. that scare med. i can't do this. a couple of days went by. my friends were like, dude, you realize that your following keeps going up? you're at 15,000. i'm like, maybe i should do something about this. i went back on and posted a lot more. >> reporter: engagingly unpolished in his many clips, his sister aaliyah as his sidekick. ♪ you'll never get your blessing till the day i die ♪ ♪ >> reporter: sean became a vine sensation. >> weird having people online thinking i'm a superstar. then going to school and everyone treating me so normal. a weird complex. >> reporter: island signed him last year. he left town and hit the road. ♪

♪ be the life of the party >> as soon as i signed to the label everything came off. "life of the party" went to number one. things got crazy. >> reporter: on a torrid pace of live performance and promotion he's been opening for taylor swift. >> she got me on stage and had 60,000 people sing "happy birthday" to me which was nuts. so nervous, my heart was pounding. >> reporter: he heeded the counsel of his greatest musical ininfluenza, ed sheer han, whose career got a boost from opening for taylor swift. >> he said to me, do everything. he said, do everything. go to every radio station, meet everybody, do everything. why not? >> this is how you do an awards show and an actual show in the same day. coffee. >> reporter: it's a life so uncommon that, despite since denied reports that he's dating camilla caballo from the vocal group fifth harmony, he admits to personal isolation.

>> it's true. feeling lonely in a crowded room is one of the realest things i can think of. it kind of happens. no matter how many people are around you. sometimes you don't know those people at all. >> i guess that's my question. who can possibly understand what your last two years have been like? >> i don't know. justin timberlake? maybe when he was my age? >> reporter: even now, mendes is so green that he has no preshow ritual. >> i'm going to chal by myself for an hour, bye, love you. >> reporter: and still gets anxious alongside his pop music peer group. >> to this day i get nervous in front of taylor and i've been on tour with her for weeks. i can't be calm in front of her. >> breathe a little bit, put it together. >> it doesn't work. she's taylor swift. she's too gorgeous. >> reporter: words on the minds of the fans who scream as sean takes the stage. >> for old times' sake. have a good show. >> if you can hear my heart right now, it's so fast.

♪ oh whoa whoa whoa >> reporter: after tonight sean will fly to canada and again live with his parents in that old house. >> we built a new deck and i was able to buy a hot tub for my mom and dad to chill in. as a kid i wanted -- i want to get a job and make money and make my parents less stressed. which made no sense, how would that work? and it kind of came true. ♪ you watch me >> reporter: for "nightline," chris connelly in los angeles. ♪ next, pope mania overtakes manhattan. thousands lining the streets just to get a glimpse of the man himself. and a waxed statue as well. but i couldn't do it on my own. i needed help and chantix was there. and i did it.

along with support, chantix (varenicline) is proven to help people quit smoking. it reduces the urge to smoke. some people had changes in behavior, thinking or mood, hostility, agitation, depressed mood and suicidal thoughts or actions while taking or after stopping chantix. some had seizures while taking chantix. if you have any of these, stop chantix and call your doctor right away. tell your doctor about any history of mental health problems, which could get worse or of seizures. don't take chantix if you've had a serious allergic or skin reaction to it. if have these, stop chantix and call your doctor right away as some can be life-threatening. tell your doctor if you have heart or blood vessel problems, or develop new or worse symptoms. get medical help right away if you have symptoms of a heart attack or stroke. decrease alcohol use while taking chantix. use caution when driving or operating machinery. most common side effect is nausea. i never thought i would be a non-smoker and i'm so proud. ask your doctor if chantix is right for you.

finally, pope francis made his new york city debut today. it's the second stop on his historic u.s. tour. after a very busy visit to washington, d.c. where he posed in front of the washington monument with president obama and gave an address to congress speaking about the american dream. >> god bless america. >> reporter: he arrived in nyc just hours ago where adoring crowds lined the streets, their cheers filling the air along his route to st. patrick's cathedral. even a papal imposter cause d a ruckus, this box pope creating quite a stir and photo ops for the masses. we'll have continuing coverage of the pope's trip to america starting with "gma" first thing in the morning.

thank you for watching abc news tonight. as always we're online 24/7 on our "nightline" facebook page and abcnews.com. thanks for watching and good night. come on. deb! what? i can't see. i hate this side of the bed. i can't see the tv from here. what do you mean? you picked that side. i dn't know it was gonna be my side for life. i gotta watch tv with a big foot in the middle of the screen. come on. i'm trying to get the weather. i can only get from wednesday over.

do you want to switch sides? what, after 7 years? i'll be all outta whack over there. maybe you'llo better with your next wife. my next wife isn't gonna have feet. that's right. i'll be able to watch tv, and i'll never have to go dancing.