Why This Attorney Believes Illinois Man Is Innocent of Murder, Who She Thinks Really Killed Missing Teen

— -- A high-profile attorney believes she has a trail of evidence potentially so explosive that it could overturn an Illinois man’s murder conviction and free him from prison.

Kathleen Zellner, who represented Ryan Ferguson when he was exonerated and released from prison after being wrongfully convicted of murder, is currently working on 31-year-old Mario Casciaro’s appeal.

“I believe we have an excellent chance of this,” Zellner told “20/20.” “This'll just be reversed outright.”

Casciaro is serving a 26-year prison sentence for the murder of missing teen Brian Carrick. The 17-year-old was last seen on Dec. 20, 2002 at Val’s Foods, a grocery store in the small town of Johnsburg, Illinois, where he and Casciaro both worked as stock boys.

Casciaro was twice tried in connection to Carrick’s death, even though his body has never been found. Casciaro’s first trial in 2012, where prosecutors charged him with first-degree murder with intimidation and unlawful restraint, ended in a hung jury and was declared a mistrial, but when he was tried a second time in 2013, he was found guilty of of first-degree murder with intimidation.

“I sit every day in a five-by-ten concrete box thinking ... ‘How did this happen?’” Casciaro told ABC News’ “20/20” in an exclusive interview from Menard Correctional Center, Illinois’ largest maximum security prison.

Zellner said a former employee from Val’s Foods, which was co-owned by Casciaro’s father, has come forward with an explosive claim that could rock the prosecution’s case.

“The new witness said that [another Val's Foods employee Robert] 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,” she said.

The witness’s statement is one of several that Zellner accuses the Johnsburg Police of not turning over to the defense during Casciaro’s trials.

According to Zellner, much of the evidence from the case leads to Render, who was also working as a stock boy at Val’s Foods when Carrick disappeared. Render’s blood was found several places in the store’s produce cooler, where prosecutors believed Carrick was killed, including his bloody finger print on the cooler door, Zellner said.

“The only two people who left blood at the scene are the victim, Brian Carrick, and Rob Render,” she said.

In an interview with police before Casciaro's first trial, Render denied knowing what happened to Carrick that night.

“I didn’t do anything. I don’t know anything about that night,” Render can be heard saying on the videotape of the interview obtained by ABC News. “Maybe I cut my finger. Maybe I bit my nails, and that’s what left blood on the door.”

Zellner said there’s no way that the amount of blood from Render found in the produce cooler could have been left by biting his nails.

“You'd have to be a hemophiliac. You'd had to have a clotting disorder,” she said.

Another important piece of evidence Zellner’s investigators learned was the existence of a pair of soiled men's underwear stained with a color that she believes is blood. A report from the investigation says a pair of underwear "soiled with a brownish red color" was turned over to Johnsburg Village Police just a month after Carrick's disappearance, but, Zellner said, it was never entered into the evidence log and was not used in trial. Zellner has asked that prosecutors locate the underwear so it can now be tested.

“That was a huge bombshell, and that's why we want to have DNA testing," Zellner said. "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."

Former Val Foods’ employee Jacob Kepple told “20/20” Render and Carrick argued over drug money that night Carrick vanished.

“Brian had told Rob, ‘If you don't have it by Friday, it's 60,’” Kepple recalled. “And to me, that meant, ‘If you don't have the $30 you owe me for weed by Friday, then it's double.’”

And, Kepple said, Render went missing for nearly two hours.

“Probably somewhere starting around 5:15 through 7:00 ... That was the first instance I'd seen him. He was standing outside the mop room when I saw him,” Kepple told “20/20.”

Render told police during his interview that his co-workers probably were unable to find him because he had smoked marijuana.

“That’s all I did. But I never left the store that night and I don’t know why people couldn't find me. Everybody is pointing a finger at me, and I don’t know why,” Render can be heard saying on the police interview tapes.

Kepple said he next saw Render by the produce cooler where there was a large pool of water on the floor. Later, he said the puddle was gone and that there were mop streaks outside the cooler. Investigators later determined that the puddle contained a mixture of water and Carrick’s blood.

Zellner believes the scene was cleaned up twice. The first was the evening of Dec. 20, 2002, the night Carrick disappeared, and then again the next morning when Mario Casciaro’s father Jerry Casciaro noticed the puddle and thought it was fruit punch.

"In my opinion it was, like somebody, you know, spilled something," Jerry Casciaro told "20/20."

Police did charge Render with concealing a murder but quickly dropped the charge. His older sister Mindy Lindholm told "20/20" her brother is innocent.

“The premise that my brother could have possibly killed Brian over $30, or over him telling on him, or anything, is ridiculous to me,” Lindholm said. “My brother having an explosive temper is absolutely ridiculous.”

Lindholm said that when she asked her brother about Carrick’s disappearance, he said he didn’t know anything about it. “He said he wasn’t there, and he didn’t know what happened,” she said.

Render was never called to testify at Casciaro’s first trial in 2012, and died of a heroin overdose that same year before Casciaro went to trial for a second time.

In another development in Casciaro’s case, this week Shane Lamb, the prosecutor’s star witness for both of Casciaro's trials, recanted his testimony that helped put Casciaro behind bars for Carrick’s murder.

“Mario didn't have anything to do with this. He doesn't deserve to be in prison,” Lamb told “20/20” from prison where he is currently on burglary charges.

Although they denied a request for an interview citing the ongoing case, the McHenry County state's attorney's office provided a statement to "20/20" about Lamb's recantation: "Shane Lamb gave a videotaped, recorded account of the incident as it occurred in Johnsburg the day Brian Carrick disappeared. He gave a videotaped recording in the State’s Attorney’s Office, with the advice and counsel of his attorney and in his attorney’s presence on January 20, 2010. He consistently repeated the same account of the events at two subsequent jury trials."

“It makes perfect sense to me that the explosive criminal in this case is Shane,” Lindholm said. “Who had such a temper that they would lose it and hurt somebody over such trivial matters? Shane. Who would shoot somebody? Shane. Who would hurt somebody? Shane. Not Mario, and not my brother.”

In regards to the underwear, assistant state’s attorney Michael Combs told “20/20” in a statement today that “The police report – which was disclosed to Mr. Casciaro’s trial counsel in 2010, indicates that the soiled underwear was placed in an evidence locker. That underwear does not appear on the evidence log and there is no record that it was ever tested or retained in evidence. The police reports tendered to counsel prior to trial indicates all of this information was presented to the defense and they could have introduced this information to the jury if they so chose.”

The Carrick family, their attorneys and the Johnsburg Village Police Department declined ABC News’ requests for an interview or comment for this story.

This week, Zellner filed a petition to have Casciaro’s conviction overturned. She said just because Render is dead doesn’t mean he can’t be held responsible for Carrick’s murder.

“The only person that really dislikes Brian Carrick is Render. The only person … that owes money to Brian Carrick is Render,” Zellner said. “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.”