Retired police officer Jack McCullough was convicted of killing a little girl in Illinois more than half a century after the crime. But he was released on Friday after a prosecutor found he could not have done it, and McCullough's conviction was overturned.

McCullough said he will sue the state for the suffering from five years of imprisonment.

Now, one of America's oldest cold cases is re-opened again.

The murder of 7-year-old Maria Ridulph occurred nearly 60 years ago, but McCullough wasn't convicted until 2012, reports "48 Hours" correspondent Erin Moriarty.

The 76-year-old sighed heavily before leaving court a free man.

"I was thinking, 'I'm going back to prison.' Then he all of a sudden went the opposite direction, and God bless him," McCullough said after his release.

The decision came after the DeKalb County state's attorney said a review of the evidence, including newly uncovered phone records, confirms McCullough was miles away when Ridulph's kidnapping occurred.

"This new information is information that the trial court did not have available to consider when it made its ruling," Judge William P. Brady said in court.

In 1957, Ridulph disappeared after a stranger approached her and her 8-year-old best friend, offering them piggy back rides. Ridulph's body was found five months later. McCullough was an early suspect but had an alibi.

"I was in Rockford, 40 miles away," he told "48 Hours" in March 2013.

The case went cold for five decades, until McCullough's own mother appeared to make a stunning deathbed declaration.

"She grabbed my wrist in the strongest grip. She said, 'Those two little girls and the one disappeared. John did it. And you have to tell someone,'" said Jan Tessier, McCullough's sister.

The new investigation revealed another possible crime: one of his sisters said that McCullough had raped her as a teenager. He was acquitted of rape, but later stood trial for Ridulph's murder.

Unable to present his alibi evidence because most of his witnesses were dead, McCullough was found guilty.

"Is it possible then that you were acquitted of what you did do and convicted of what you didn't?" Moriarty asked McCullough in 2013.

"That could be," he responded.

"Is that what happened?" Moriarty asked.

"Don't know," McCullough said. "I may have been a sinner, but I'm not a murderer."

McCullough hasn't been exonerated, but his conviction has been overturned. That means he could technically be retried, though it seems unlikely.