Star Witness in Illinois Missing Teen Murder Trial Recants Testimony That Sent Co-Worker to Prison

— -- A former Illinois grocery store worker, who was the star witness in a missing person’s murder trial, is now saying prosecutors coerced him into giving false testimony to help put an innocent man behind bars.

“All of it was false, every single thing ... the state’s attorney set it up,” Shane Lamb told “20/20” in an exclusive interview from prison where he is serving time for burglary charges.

Lamb was working as a stock boy at Val’s Foods, the only grocery store in the small town of Johnsburg, Illinois, when another stock boy, 17-year-old Brian Carrick, went missing Dec. 20, 2002.

Carrick hasn’t been heard from since and his body has never been found. But in April 2013, another former stock boy, Mario Casciaro, 31, was convicted of first-degree murder with intimidation in Carrick’s death and sentenced to 26 years in prison.

But Lamb, whose testimony was a crucial part to the prosecution’s case against Casciaro, is now claiming neither he nor Casciaro had anything to do with Carrick’s death or disappearance.

“I didn’t have anything to do with this,” Lamb told “20/20.” “Mario didn’t have anything to do with this. He doesn’t deserve to be in prison.”

Casciaro’s family owns Val’s Foods, and several members of the Carrick family, who lived across the street from the store on Johnsburg Road, worked there.

When Brian Carrick disappeared, police later found blood evidence belonging to Carrick in the store’s produce cooler. They also found a bloody fingerprint on the door handle. Authorities were convinced that Carrick was killed over a drug debt he owed Casciaro, who was working at the store the day he disappeared. But with no witnesses or physical evidence linking Casciaro to the crime scene, the investigation went cold.

For years, Lamb denied he knew anything about Carrick’s disappearance. But then, in 2010, he said he was again facing serious charges and made a deal with assistant state's attorney Michael Combs.

“I was arrested for cocaine charges and my offer was 12 years [prison time],” Lamb told “20/20.” “They said that I would be indicted for murder if I didn’t cooperate.”

In a 2010 videotaped meeting with prosecutors obtained by ABC News, Lamb says on the night Carrick was last seen, Casciaro had called Lamb to ask him to come to the store to scare Carrick into paying back drug debts he owed. Prosecutors characterized Casciaro as a drug dealer and Lamb as his enforcer.

Lamb is heard on the tapes telling prosecutors he punched Carrick a few times and left him unconscious in the produce cooler. He claimed that Casciaro then told him, “Get out of here. I’ll take care of this.”

In exchange for incriminating testimony against Casciaro, Lamb said prosecutors offered him immunity from all charges related to Carrick’s death and a reduced sentence on a cocaine conviction.

Casciaro was charged Feb. 25, 2010, with first-degree murder with intimidation and unlawful restraint in Carrick’s disappearance, a rare charge that meant even though Casciaro never touched Carrick, or ordered Lamb to hurt him, he was still responsible for Lamb’s actions that night.

Prosecutors took Casciaro to trial twice. At the end of the first trial, on Feb. 1, 2012, jurors were deadlocked, 11 to one in favor of the prosecution, and a mistrial was declared. The case was retried in March 2013, and on April 2, 2013, Casciaro was found guilty of first-degree murder with intimidation.