Illinois Man Serving 26 Years in Prison for Murder He Says He Didn't Commit

— -- The last time Mario Casciaro says he saw 17-year-old Brian Carrick was in Val’s Foods, the Johnsburg, Illinois, grocery store co-owned by Casciaro’s father. Both Carrick and Casciaro worked as stock boys. It was a Friday night around Christmastime.

“I saw Brian once at the beginning of my shift,” Casciaro told ABC News’ “20/20” in an exclusive interview. “Brian was looking for Rob Render [another stock boy] and had asked me if I'd seen him, and I paged him, and that was the last time I seen him. That's it.”

That was that last time anyone saw Carrick, who has now been missing for 12 years.

But even though Carrick has never been found, Casciaro is currently serving a 26-year prison sentence at Menard Correctional Center, Illinois’ largest maximum security prison, for the murder of the missing teen. But Casciaro maintains he had nothing to do with his death or disappearance.

“I sit every day in a five-by-ten concrete box thinking ... ‘How did this happen?’” said Casciaro, 31.

Brian Carrick disappeared on Dec. 20, 2002. That day, Casciaro said he had a routine shift at the store, and afterwards, he picked up a pizza and shared it with Brian’s brother Eddie Carrick, who also worked at the store, and others in a break room. That evening, Casciaro said he went to a party at a friend’s house, where he said he spent the night.

The store closed at 8 p.m. and security camera records indicate no one entered or left the store after closing until the next day.

The Carrick case shocked the small town. Grisly rumors began circulating that Carrick was believed to have been killed in the grocery store’s produce cooler, but eventually, the investigation went cold.

For Casciaro, life moved on. Three years after Carrick’s disappearance, he received his degree in finance from Illinois State University and joined his dad in expanding the family business.

But then, in 2010, Shane Lamb, a five-time convicted felon with a rap sheet that included an attempted murder charge he got when he was just 14, gave prosecutors a shocking confession that led to Casciaro going to trial.

Lamb had also worked as a stock boy with the Carricks and Casciaro at Val’s Foods in 2002. For years, he denied to authorities knowing anything about Brian Carrick’s disappearance, but when he faced 12 years in prison on cocaine charges in 2009, he made a deal with prosecutors.

In a 2010 meeting with prosecutors, Lamb told prosecutors that on the night Brian Carrick was last seen, Casciaro had asked Lamb to come to the store to scare Carrick into paying back drug debts he owed, according to videotape of the meeting with prosecutors obtained by ABC News. 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 the night of Dec. 20, 2002, leaving him unconscious in the produce cooler. He claimed that Casciaro told him to leave and that he would take care of Carrick.

But Casciaro said he barely knew Lamb. At the time, Lamb had just gotten out of juvenile detention and was hired as a stock boy just months before Carrick disappeared.

“We weren’t friends that hung out on a regular basis at all,” Casciaro told “20/20” of Lamb. “I wasn't even in the produce area, and put yourself in that shoes. Why would I say, ‘Yeah, buddy. Let me take care of this for you?’ I don't even know that kid.”