reporter: tonight, on "20/20" -- no body, no weapon, but definitely a murder. >> this is a very quick crime scene, and then a frantic cleanup. >> reporter: and a bombshell jailhouse confession, that could change everything. >> i'm not going to have that on my chest anymore. >> reporter: what really happened to popular teenager brian carrick, who walked to work in a grocery store one night, then just vanished? >> not knowing where he is, a parent's worst nightmare. i guess we're living it. >> reporter: with a revolving door of suspects, stock boys with a past, is the right one behind bars? >> there's somebody who knows exactly what happened. >> the "20/20" exclusive with the man locked up for murder. out of shackles, to talk only to us.

>> what did you think that circular stain was from? >> the secret, the interrogation tapes, stock boys on the hot seat. >> i didn't do anything. i don't know anything. >> and a blood sniffing crusader who says she can name the real killer and show what he did. >> comes up behind him and cuts his throat. >> tonight, can you solve the mystery on johnsburg road. >> where did he go? people don't disappear. now elizabeth vargas and david muir. >> good evening and tonight here, a mystery in the heartland, a popular teenager who leaves his house to go to work across the street but never comes home and is never found again. 12 years later he is still missing, someone is behind bars tonight for the crime, but the question, is he really the killer. >> get ready for some bombshells because everything in the mystery on johnsburg road is about to be turned upside down.

evidence goes missing a prisoner says he lied, and the key witness turns up dead. who says the dead keep all their secrets. here's abc's ryan smith. >> reporter: the village of johnsburg, illinois, the essence of small-town america, a flag-waving heartland some 50 miles outside of chicago, a conservative community where tradition reigns and nostalgia is embraced. some call this "mayberry of the midwest." >> there's a lot of farms out there, still. and then there's some subdivisions that have crept in. you've got the grocery store, bowling alley. very simple, very, just kind of easy. >> reporter: but johnsburg is also a village with a secret, a lingering 12-year-old unsolved mystery that still has people talking. >> i can't really think of a bigger news story. >> i would say everybody had an opinion. >> i think there's somebody who knows exactly what happened.

>> reporter: what happened to brian carrick, a likable 17-year-old high school junior, who one friday night vanished, from inside the grocery store where he worked as a stock boy, just five days before christmas. amanda marrazzo covered brian's disappearance for "the chicago tribune." >> he was excited about christmas. there was no indication for him to run away. >> reporter: his father, william carrick, an electrician, says his youngest son was always by his side. >> wherever his dad went, his dad said he'd be in his hip pocket. he was an entrepreneur, started making money very early on shoveling snow. >> reporter: and he loved val's foods, the village's only grocery store, directly across the street from the carrick home. val's co-owner, jerry casciaro. >> he was smart that you have no idea how smart he was. if you tell him to do one thing one time, say this is what i want you to do, next time, he'll tell you what to do. >> reporter: so he was a go-getter? >> if there were more people like brian in this country, there will be a beautiful future

for america. >> reporter: in johnsburg, everyone knew the carricks, the hardworking irish catholic family with 14 children. brian, the 11th. >> the parents, when they got married said "god, give us as many kids as you want to give us, and we'll take them." >> reporter: describe the carrick family for me. what were they like? >> all very nice people. one of the sisters said they didn't have the best clothes, and they didn't always have school supplies, but they had a lot of love. >> reporter: that december 20th was brian's day off, but went to val's looking for a co-worker, passing his brother eddie, also a stock boy, on the way. >> eddie was going out to get the carts from the parking lot and brian was walking into the store. he never saw his brother after that. >> reporter: the next day, his mother, terry carrick received a troubling phone call. brian hadn't shown up for work. >> she knew her son would never miss work, that was his life was going to that store. she went upstairs. he wasn't in his bed. she knew immediately something's wrong.

>> reporter: within days, johnsburg was reeling in disbelief. supporters gathered at a vigil for brian, walking the same route he walked to work across johnsburg road. his mother terry, the strong matriarch shaken. >> it's every parent's worst nightmare and i guess we're living it. >> no one knew if brian was dead or alive. >> illinois state police became involved. the fbi became involved. psychics would come to terry front door. >> val's foods posted a $25,000 reward. the grosser's daughters julia and diane. >> somebody would say they're putting up $25,000 for someone

who works for them, not a family member but an employee. >> the carricks needed it. they didn't have the money. we were desperate to try and find what happened to him. >> the carricks and the casciaros, two of johnsburg's most well known families, friendly for nearly 20 years. almost half the carrick kids worked at val's. >> >> i understand you have a picture of him. >> definitely. >> can you show me? >> 12 years later you're still carrying a picture of this kid in your wallet. >> right. >> why do you carry miss picture with you? >> to me he was like a son. >> an early break in the missing persons case revealed blood evidence in and around the grocery store produce cooler, news no one wanted to hear, drops, spatter, smears, a bloody fingerprint, brian's blood. what mystery did the grocery store hold? >> what was your reaction when

you heard the police found blood in the cooler? >> it was immediately who did it. >> everybody was on edge. >> police questioned everyone, including three stock boys, shane lamb, rob render, and mario casciaro. two of them have criminal records. but police quickly turned their attention to the one would no priors, the boss's son, 19-year-old mario. jerry's heir apparent. >> are you kidding me? >> you guys were shocked at that point. >> yes, completely shocked. mario is really one of the nicest, kind, intelligent, funny, just a sweetheart. >> cops were beginning to develop a more sinister profile was brian's father, william carrick, would share with a local reporter. >> mario allegedly was selling dope, and i think he coerced my son into working for him.

somehow, yeah, things got out of hand. >> there were rumors that there was some things going on at the grocery store that shouldn't have been going on. >> reporter: tell me about that theory. >> the story is that mario casciaro was selling drugs and that he would, you know, use some of the kids from the grocery store, like brian carrick, to sell the drugs. and brian carrick, being the sweet kid that he is, wouldn't always collect the money. >> reporter: authorities were convinced that the boy who freely gave away pot was a victim of foul play. they believed he was killed over a $400 to $500 drug debt oh, owed mario. the police began to dig into mario and little bit and they believed that he was selling drugs. >> i think, you know, maybe he smoked pot. i think maybe he, him and his friends bought it and sold it to each other. i mean, we're talking about a very small amount. >> reporter: but with no eyewitnesses, no physical evidence tying mario to the crime scene or even a body, the investigation grew cold and so did the bond between the

carricks, and the casciaros. >> the relationship had gone sour. and then, one day, my dad said that mrs. carrick had come in the store and she kind of turned her back to him when he approached her to say hello and that was the last time she ever spoke to him. >> reporter: but for mario, life was moving on. here he is receiving his degree in finance from illinois state university, three years after brian's disappearance. but for the carricks, no closer to any answers, it was as if time stood still. mario joined the family grocery, worked as a manager, and helped build the business. >> that was not the only store that he was going to have. it was always going to be two stores, and three stores and four stores. >> reporter: that grand plan would never happen. mario's world was about to collapse around him. a break in the case. this stock boy, shane lamb, would change everything with a tale of violence in the produce

cooler the night brian vanished. >> i hit brian a few times. he was bleeding from his mouth. i thought i knocked him out. >> where's mario when you're doing this? >> right in the doorway. >> reporter: what did happen to brian carrick? stay with us. an iphone 6!!!!!! ! it covers facetiming? uh-huh. noice. what's that all about? i'm streaming!!!!! it covers that? oh-yeah. way to go. omg. blasting this!! it covers blasting? sure does. word. what are you doing sweetie? posting pics. it's covered. we are awesome parents. get iphone 6 on straight talk wireless. because you need a plan that covers that... without costing all that. $45 unlimited with the first 3 gigs of data at high speeds then 64 kilobytes per second. walmart. --i don't know my credit score. that's really important. i mean - i don't know my credit score. don't you want to buy a house...like, ever? you should probably check out credit karma, it's free. credit? karma? free? credit karma. really free credit scores.

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>> announcer: "20/20" continues. once again, ryan smith. >> i would like to thank everybody for attending tonight. a toast to prosperity and the future. salud. >> reporter: happy memories for mario and the casciaro clan, older brother eugene's big italian wedding. but good times were in short supply for this family.

jerry and maria casciaro, italian immigrants, met and married in the states, realizing their american dream when they joined with relatives to buy val's foods. >> you know, he didn't have anything given to him. he came to this country, built what he had from the ground up. >> reporter: in an age of superstores, val's thrived in the small town where a night out is spent at a bowling alley or a pizzeria. nearly every family in the village shopped there, so many of the local kids worked there, and for the casciaro's, the future was bright. those dreams, now tarnished by tragedy and grisly rumors about missing stock boy brian carrick, believed to have been killed inside the store's produce cooler. >> one of the persistent stories is that brian was killed in the cooler. mario called family from the city who came in, dug him up, dismembered him and threw him in

a river in iowa. >> it snowballed into these disgusting rumors that he was cut up in our meat department. and then, it started becoming that our family was involved in some kind of crime family. >> reporter: the fbi investigated and dismissed rumors of a mob hit. this case may have remained an unsolved mystery if not for this man. >> this is assistant state's attorney michael combs. >> reporter: also known as mike combs, a tough, hard-edged prosecutor, determined to find justice for the carrick family. 2010, a break in the eight-year-old case. prosecutor combs finds a key witness. his name, shane lamb. who is shane lamb? >> shane lamb. he's got a criminal history. he grew up rough. i think he's a very tragic character in all of this. >> reporter: a stock boy at val's who worked alongside brian and mario, a five-time felon with a rap sheet that included

attempted murder at just 14. he'd repeatedly denied knowing anything about brian's disappearance to authorities. now, eight years later, in jail facing up to 12 years on drug charges, he was ready to talk. but first he wanted a deal and so did prosecutors. >> shane lamb is given immunity from the offense of murder, involuntary manslaughter, concealment of a homicidal death and any other offense resulting in his involvement in the disappearance of brian carrick. >> reporter: with this deal, shane escaped all charges related to brian's death and a reduced drug sentence. all he had to do to get the deal, tell his story and that led police to mario. >> what exactly did mario tell you? >> pretty much talk to brian, try to get the, intimidate him into getting the money. >> how were you to intimidate him? >> talk to him. he never came out and said, "go kill him," or anything like that. >> reporter: a controversial deal awarded to a man who admits he was the likely killer, and mario was the mastermind. >> according to the prosecutors,

mario knew what he was doing by bringing shane lamb in and using him as a tool, as his enforcer, as the muscle, to get what he wanted. >> reporter: shane was off that night at a party getting high. he says mario called him to come to the store. brian owed him money, and he wanted it back. mario wanted shane to talk to him. it was time for him to pay up. >> i went over there, told brian, "what's up with the money you owe him? why don't you pay him at least some money back?" we got to arguing. mario said it was getting too loud, go in the produce cooler right there. >> reporter: shane says he demanded the money from brian, but brian resisted. shane lost his temper. things turned violent. >> i hit brian a few times. he was bleeding from his mouth. fell out. >> how hard did you hit him? >> i thought i knocked him out. that's it. >> where's mario when you're doing this? >> right in the doorway. >> reporter: knocked unconscious, not sure if brian was dead or alive, shane says mario told him to leave the grocery store. >> and he said, "don't worry. i'll handle this." i left and went back to the party.

>> reporter: armed with shane's version of what happened, prosecutors arrest mario for killing family friend brian carrick, a small victory for bill carrick, reminded daily of the store across the street on johnsburg road, a moment terry carrick would not live to see when she died in 2009. for the casciaro family, it was a moment of utter disbelief. >> i wanted justice for the carricks, but they got the wrong guy. >> reporter: mario and shane, the mastermind and the muscle, according to the prosecution's theory. though shane says mario never laid a hand on brian, never ordered him hurt, prosecutors insist he's guilty of the rare charge of murder with intimidation. to them, just by uttering the words "talk to him," mario casciaro sicced a weapon of destruction on brian, and was responsible for everything that followed, including brian's murder.

had you ever heard of this kind of charge before? >> no. it's very rare. >> reporter: had it ever been used before? >> from my understanding, it hasn't. >> reporter: an aggressive move, the charge never before used in illinois state history. mario plead not guilty. the case goes to trial. after 12 hours, jurors were deadlocked, eleven to one, in favor of the prosecution. a hung jury. >> sometimes lay people struggle with the one person being legally accountable for the actions of another. >> reporter: when you learned that it was a hung jury, what were you thinking? >> we were frustrated just like the carricks were. >> reporter: prosecutors went after mario again, vowing to reply him. mario goes back to court facing another murder trial and once again, shane lamb is the prosecution's star witness. prosecutors telling jurors shane lamb was mario's "enforcer," a tool for his "dirty work." >> you don't bring mother teresa to a shakedown. you bring a bad ass like shane lamb. >> reporter: in closing

arguments, prosecutors branding the pair "mean little delinquents," "criminals who flock together," in mario's "drug cartel." so this is a narrative the prosecutors are building so that people can see mario as some dangerous, nefarious figure? >> well, i mean, if you have evidence, then you stick to the facts and you say, "okay, here's his dna. here's his blood. here's, you know, the timeline. we didn't have any of that. >> no. >> so they had to dirty him up a little bit. >> reporter: prosecutors' clever strategy worked. >> he kind of looked more like the spoiled rich kid that was trying to be like a little mini-gangster or something. >> reporter: the man who didn't say a word throughout two murder trials decided to speak only to "20/20." but why now? did you kill brian carrick? stay with us. gets you cleaner, but cottowill it make peopleexture confident enough to go commando? how was your wiping experience?

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to show exactly what you need. life's the food that brings us together. and kitchens where every meal is the most important of the day. >> announcer: "20/20" continues with "the mystery on johnsburg road." here, again, ryan smith. >> reporter: tonight, mario casciaro sits inside one of the toughest maximum security

prisons in the country, menard. over a century old, the notorious and the deadly have been here. remember him, the serial killer clown, john wayne gacy? he did his time here. and now, it's home for mario casciaro, which is where william carrick believes he belongs. >> everything i've heard about these maximum security prisons is bad. i guess he's earned his place. >> reporter: the college graduate and budding grocery store magnate now a convicted murderer shackled and chained to the floor. brian carrick's family believes that you belong here. >> if allegations are repeated over and over and over again for, say, seven years, you begin to believe that it's a fact. >> reporter: the carricks believe that you know where brian's body is. >> it's sad that they think that. it's sad that they would ever think something like that and i really hope that we find brian.

>> reporter: a toast to mario before every family meal. >> the welcome of mario in the very few future. >> i'm not a criminal. nobody in my family's ever even been in handcuffs. i'm the first and that includes extended family as well. >> reporter: mario says brian carrick was a loyal employee and they had a good relationship. >> he was a good guy. good family. he worked hard. one of my favorite co-workers. >> reporter: 12 year later and mario still remembers december 20th, 2002. just five days before christmas, the store packed with shoppers filling their carts with all the makings of christmas dinner. it was brian's day off, but he showed up that evening around 6:30, asking for another stock boy, robert render. >> brian was looking for rob render and had asked me if i'd seen him. and, "hey, where is this guy?" and i paged him, and that was the last time i seen him. >> reporter: after seeing brian, mario says he picked up a pizza

nearby, sharing it with employees in the break room clear across the other side of the store from the produce cooler. that's his alibi. mario says he helped close the store, as usual, at 8:00. both the defense and prosecutors believe brian was murdered before closing time. did you kill brian carrick? >> absolutely not. >> reporter: are you responsible in any way for his death? >> no. >> reporter: as for the prosecution's claims that the man who put him in prison with his testimony, shane lamb, was called to the store that night to talk to brian? so this is important because police believe at some time you call shane lamb. did you ever call him? >> no. we actually gave them my phone records and showed them that there was no call. >> reporter: shane, on your urging of, "talk to him," has an altercation with brian carrick, punches him a few times, lays him out. i -- you tell shane, go, i'll take care of this.

any truth to that? >> not at all. i didn't even see shane in the building that evening. why would i say, "yeah, buddy. let me take care of this for you." i don't even know that kid. "let me just take care of a murder for you." be serious, you know what i mean. >> reporter: he questions why anyone would believe he'd take a murder rap for shane lamb, who'd only worked at the store for a few months after a two-year stint in juvie for attempted murder. but what about the witnesses who testified mario sold pot. did you ever sell drugs? >> there were times that, when i was smoking pot, that i would, i guess, sell people pot out of my personal stash. but it wasn't anything that i ever, it wasn't, like, a criminal enterprise. >> reporter: did brian ever sell pot for you? >> no. >> reporter: did brian ever owe you money for selling pot? >> no. no. >> reporter: so this claim about him owing you $400 or $500 and that's why you had shane talk to him. >> it's all made up. he took something that's very small, small amounts of marijuana, and made it seem like it was a huge criminal enterprise. they actually went so far as to say i was a drug cartel and

kingpin. that's significantly different than smoking some weed with your friends. >> reporter: former local reporter sarah strzalka agrees. >> there were multiple people who got up on the stand and said that they had bought, you know, pot from brian or bought pot from mario, or whatever it may have been. but this wasn't a cartel. it wasn't some big drug empire. it was high school kids buying a little bit of pot here and there. >> reporter: you didn't testify at your trial. how do you feel about that decision now? >> i think it was the wrong decision. i wanted to do it at the time. i was just advised not to. >> reporter: his future was in the jury's hands and they never heard from him. that may have cost mario his freedom. the verdict. what's going through your mind as you're waiting for that verdict in the second trial? >> "what is taking so long? what are they thinking about? what could they possibly have found to be credible that they've seen?" >> reporter: did you think, at that point, you were going home with your family? >> yeah. sure did. >> reporter: how firm was your

belief that you were going home? >> 99. >> reporter: going home wouldn't be in the cards. a guilty verdict, mario's mother's outrage caught by local news cameras. >> my son is innocent. there is a cover-up, big time. >> his family sobbed. his father wailed. mario was confident the whole time, his head was up and then he just drops his head. he was shocked, thought he would never be convicted. there was no physical evidence. >> it was heart-wrenching. it was if mario had died. >> reporter: can you make it through 26 years in here? >> i don't want to. i mean, if i have to, i will find a way, but i don't want to. but i don't want to sit here every day thinking about how did i get here because it's -- i don't have the answers, you know what i mean? >> reporter: enter this woman. she's uncovered a detail never brought to trial, something hidden behind a ceiling tile.

>> announcer: next, a new suspect enters the picture. >> i didn't do anything and i don't know anything. >> announcer: as we follow a trial of blood and the trail of a knife. >> render comes up behind him like this and cuts his throats. >> announcer: defense with a dummy when we come back.

>> announcer: once again, "20/20," and ryan smith. >> reporter: they walk in silent protest, a family crusade to free 31-year-old loved one, mario, sentenced last november to 26 years for murder. >> there was no physical

evidence, no dna, no witnesses and no motive. >> when you are innocent, you are going to fight until the very end because you know you're innocent. and we're going to fight till the very end 'cause we're 100% behind him. >> reporter: and that's why they hired this gladiator, kathleen zellner, ahigh-powered attorney, to appeal his case. her reputation, overturning convictions and solving cases. she's uncovered what she believes are secrets long tucked away inside the family grocery store. >> i believe we have an excellent chance. this'll just be reversed outright. >> reporter: according to kathleen, the trail of evidence potentially so explosive it may set mario free. she believes the evidence leads not to mario, not to shane lamb, but to one man, rob render, an early suspect in the case, that stock boy brian accused of stealing alcohol from the grocery store, and the one brian was looking for the day he vanished.

>> what do we know about rob render? >> another troubled soul, he's just had a lot of problems, a lot of drugs. >> reporter: for kathleen, the case begins with the physical evidence. the blood at the crime scene, rob render's blood was there and his was the only blood that was ever found aside from brian's. >> we know that rob render and brian carrick had an altercation because both of their blood is right there at the crime scene. >> reporter: for her, the crime scene tells the story. look at these photo exhibits. while brian's blood is in the hallway leading to the cooler, rob render's bloody fingerprint is on the cooler door handle. and inside the door, more of rob render's blood. when police questioned rob, he insisted he wasn't there. >> i didn't do anything and, i don't know anything. and it's like, you know? >> reporter: they asked rob if he wasn't there, why was his blood? >> maybe i cut my finger. hey, who knows?

maybe i bit my nails so bad, 'cause i do, that it bled a little bit and i put my thumb on the door. >> there's no way that this amount of blood could have been left by rob render by biting his nails. you'd have to be a hemophiliac. you'd had to have a clotting disorder. >> reporter: and for kathleen, the motive, brian turned in rob for stealing booze, ridiculed him at work for being weak and, according to jacob kepple, another stock boy at the store, rob was in debt to brian for pot. >> brian had told rob, "if you don't have it by friday, it's 60." and to me, that meant, "if you don't have the $30 you owe me for weed by friday, then it's double." >> reporter: next, opportunity, where was rob render that night? kathleen contends that other employees said rob was nowhere to be found for nearly two hours. but according to rob -- >> i was probably stoned. >> that night? >> i'm sure. i was 17 years old. that's all i ever did. but i never, ever, left work.

you know, maybe they were looking for me and they couldn't find me. maybe i was stuck in an aisle somewhere, maybe, you know, maybe i went on break. i'm telling ya, i never left the store. >> reporter: but maybe he knows more than he's telling. that's what police thought. he told a friend there was a fight in the cooler and that led them to the crime scene. >> reporter: honestly, that's what led us over to the cooler and led us to the blood. >> somebody else has said that, that there was a fight in the cooler. nobody else knew where the scene of the crime was. >> reporter: a detail kathleen believes only the killer would know. she says rob also tried to cover his tracks, mopping up the produce cooler that night and being on the scene the next morning when co-owner, jerry casciaro, noticed a pool of red-tinted water. >> what did you think that circular stain was from? >> at the time, actually i only thought it was hawaiian punch. >> okay. >> i got a little hot under my collar and i said, "okay, you. clean it up." the "you, clean it up" was rob render.

>> reporter: and kathleen has a witness, an employee at the grocery store when brian disappeared, who could rock the prosecution's case with explosive claims. >> what did this new witness have to say? >> the new witness said that render 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 matches her version of events. kathleen says the fatal blow wasn't from a punch, as shane lamb confessed, but from a knife. >> tell me what you believe happened between brian and rob render. >> i believe that render comes up behind him like this and cuts his throat and in the process, cuts himself. then, i believe, that brian starts falling forward and render grabs the door. that's why his thumbprint is on the door and then he pushes him like this. then he scrapes against those boxes. that's why the transfer blood is there. the attack is in the hallway.

so we're all familiar with the blood evidence. >> reporter: and acting on a tip from this witness, kathleen's investigators, finding a piece of evidence in the police report, a pair of soiled underwear, with a brownish red color. kathleen thinks it may be blood. >> mario's attorney, kathleen zellner, believes that what may have happened in this ladies' bathroom could blow this case wide open. a stock boy, coming in this bathroom about a month after brian disappeared, saw that there was a leak in this bathroom and saw that it lead up to this panel. so he stepped on this toilet, lifted up this panel, and found a pair of men's underwear, size small. >> reporter: a size similar to rob, the underwear, handed over to the police. but kathleen says it wasn't entered into the evidence log and never made it to trial. >> that is a huge bombshell and that's why we want to have dna testing. we want to confirm if that was his underwear because if that's got render's blood on it and carrick's blood on it, that is all you need to know about who

committed the murder. >> so we don't know that for sure. >> no. >> you just need to check it out to see if that's a part of the picture? >> yes. >> reporter: prosecutor combs admitted to "20/20" that the underwear was not tested, but stated both the police report and the witness were available to mario's defense lawyer before trial. >> unfortunately for you, the evidence points at you. almost all of it points at you, rob. >> reporter: rob wasn't let off scot-free. police charged him with concealing brian's murder, but later dropped the charge. rob render never told his story in court. struggling with drug issues for years, he overdosed on a cocktail of heroin and cocaine and died in 2012, just months before mario's second trial. mindy lindholm, rob's older sister, passionately defends her brother. she claims he would never hurt anyone. >> the premise that my brother could have possibly killed brian over $30, or over him telling on

him or anything, is ridiculous to me. my brother having an explosive temper is absolutely ridiculous. >> reporter: and she accuses kathleen zellner of scape-goating her dead brother. >> i asked him, you know, what he knew about it and he said he wasn't there, you know, and he didn't know what happened. >> i'm sure prosecutors will say this is a classic case of blaming the dead guy. >> to me, that's meaningless to say, "blame the dead guy." doesn't mean dead guys didn't commit murders. the only person that really dislikes brian carrick is render. 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 and the only person in that back hallway is render. >> reporter: next, the state's star witness, shane lamb, the man who put mario casciaro behind bars, does a shocking about face. >> all of it was false, every single thing. that state's attorney set it up. >> reporter: stay with us.

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>> announcer: and now, one more exclusive interview tonight, this time with the man whose own words helped put that grocery store stock boy behind bars. tonight, right here on "20/20," he changes his story and here again, ryan smith. >> a dramatic development in the mario casciaro case. the man at the center, the state's star witness whose testimony helped send mario to prison, now says it was all a lie. shane lamb sat down exclusively with "20/20" with a stunning

confession. >> why are you coming forward now? >> because i -- mario's in there for 26 years for something he didn't do. i didn't have anything to do with this. mario didn't have anything to do with this. he doesn't deserve to be in prison. >> and it's safe to say that he's serving 26 years in a maximum security prison because of your testimony? >> yeah, that's right. >> reporter: he said he was coerced by a prosecutor hell-bent on putting mario behind bars. >> what do you want to say about your testimony today? >> all of it was false. every single thing, the state's attorney set it up. >> reporter: he's talking about assistant state's attorney michael combs, saying he forced him to place mario at the crime scene to get that immunity deal. the one to avoid being charged in brian carrick's death. shane says his back was against the wall. >> i was arrested for cocaine charges. my offer was 12 years. they said that i'd be indicted for murder if i didn't cooperate. >> reporter: and it came down to him or rob render. >> he said rob render was in lake county jail. if i didn't want to talk to them, that they're going to go to rob render next.

>> so you believed that they were either going to feed a story to rob render that he was going to tell about his involvement or mario's involvement? >> right. >> and whoever took the deal first. >> exact, yeah, 100% that's how i felt. >> reporter: here he is giving his statement. but according to shane, what you don't see before this point, before the cameras roll, he makes allegations that combs sat him down for an hour without his lawyer, telling him what to say, point by point. >> on december 20th, 2002, mario casciaro never called you to say, "come to the store and talk to brian carrick because he owes me money." >> never. >> and you weren't his enforcer? >> no. >> did you guys ever get into a fight? >> no. >> you never punched him two or three times knocking him unconscious? >> never. >> and then, mario never said to you, "get out of here. i'll take care of this." >> never. never happened. >> reporter: explosive allegations, especially if true, but there's a problem. shane lamb has a long history of lying, along with a lengthy rap sheet. >> you're a five-time felon.

you've spent a lot of your life in and out of jail. can you understand that people are going to say, well, how can we believe him? >> yo, they believed me enough to use my testimony to put him away. but now, since i'm telling the truth, now they're going to say i'm lying. >> reporter: and this, if shane is to be believed, why would a prosecutor risk so much? >> why would he put his career on the line, threatening you just to put mario in prison? >> i don't know why they -- a lot of people don't like mario for some reason in law enforcement. >> reporter: that's a question we put to mario himself. police took the stand and talked about how you were arrogant in talking to them. >> i don't feel i'm arrogant. >> do you think that you're sitting here maybe in large part because they just didn't like you? >> this isn't a popularity contest. this isn't the senior prom. this is my life. if i'm sitting here because they don't like me, then i shouldn't be here. >> reporter: the state's attorney's office wouldn't go on camera because the case is on appeal. in a statement to "20/20" today, prosecutor combs denied coaching

shane lamb out of the presence of his lawyer, calling it "unworthy of belief, untrue and too farfetched." >> you know, as i hear you today, i think, "what do you have to gain from all this?" >> i have absolutely nothing to gain. the only thing that can, only thing that can happen to me right now is them recharging me for murder. i have everything to lose right now. >> so you have nothing to gain, everything to lose. is that why you think people should believe you? >> 100%, they should believe me. it's the truth. >> how does this change your defense of mario? >> it dramatically changes it. i mean, i think, the underwear that's been discovered, the witness statement about render's plan to attack brian carrick, combined with shane lamb recanting everything, he was the whole case. i mean, he was the only person that was responsible for mario being convicted. >> reporter: but then, who killed brian carrick? all three stock boys tell a different story. shane lamb now says he was never at the store that night and no co-workers placed him there, either.

mario casciaro claims he was eating pizza in the break room. and before his death, rob render proclaimed his innocence and brian carrick's body has never been found. but for shane lamb, one thing is clear. he is willing to spend the rest of his life in prison so that mario doesn't have to. >> he's in one of the worst prisons in illinois for 26 years. they let somebody make up testimony to get him locked up. >> you were the one who put him in menard. >> i was, but i was following what they wanted me to say. they just wanted to close the case. >> it seems like this weighed heavily on you. >> it did. i don't want to be the reason that he's doing 26 years. if, you know, mario is watching this, i just want to tell him i'm sorry, his family that i'm sorry. >> if you feel so deeply about it now, where was that feeling when you were telling the story that put him in prison? >> at first, you know, i don't

care how tough you think you are, you know, it doesn't seem real until somebody gets convicted. >> i'm happy that he's finally telling the truth. i wish he would have done it at the trial so that way i wouldn't had to have been subjected to this because this changes your whole life. >> are you losing hope when you sit in here? >> it's hard to be optimistic. every day that goes by is another day that i'm not with my family. >> and god bless mario. >> my dad's 79 years-old. i don't know how much longer he has left. and every day that goes by, i'm missing out on that. >> missing out on being with your family. >> we lost brian. we're not going to get him back and then the only thing -- the only thing i can hope for is

that i'll meet him again someplace. >> reporter: two families, in pain and seeking justice for their loved ones, but has justice been served? where brian carrick is remains "the mystery on johnsburg road." >> there are some updates since we first began reporting this story. sadly, brian carrick's father william died this past december, without ever knowing his son's fate. shane lamb, who changed his story, plead guilty to an unrelated charge and faces 40 years in prison. and as of tonight, mario casciaro is still in prison at menard, his new legal team appealing his conviction. >> but a new hearing could change everything. bizarrely, that "underwear in the ceiling," which kathleen zellner thought may have contained rob render's dna, no longer exists. however, there's yet another set of clothing that was never tested taken from render's trash after brian carrick's disappearance. it's being tested for dna now,

and zellner hopes to have the results in time for a new hearing in two weeks. so the mystery on johnsburg road continues. we'll be right back. by a regist.

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>> after seeing all the twists in tonight's mystery, we want to know what you think at home. use the #abc2020. i'm david muir. >> and i'm elizabeth vargas. for all of us here at abc news and "20/20," thanks for watching, and enjoy the rest of your weekend. goodnight.

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