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A 76-year-old Washington state man released from prison after a prosecutor concluded he was wrongly convicted in the 1957 killing of a schoolgirl said Sunday he intends to sue Illinois for the suffering five years of imprisonment caused him and his family.

Jack McCullough, a former security guard and Vietnam veteran, spoke to The Associated Press by telephone for several hours during his second full day of freedom after a county judge ordered his release and vacated his 2012 conviction in the slaying of 7-year-old Maria Ridulph in Sycamore, Illinois.

McCullough described spending 23 hours a day locked in an 11-by-5-foot cell. While imprisoned, he wrote to the Illinois governor and the U.S. president pleading them to intervene. His letters went unanswered, he said.

McCullough wasn't worried when he was first arrested in 2011 because, he said, "I know I didn't do it." His conscience was clear as he entered prison, he said.

But the experience was still traumatic for him and his family, especially his wife.

"They didn't just punish me -- they punished ... my whole family," he said. "I want the state to be (held) responsible."

McCullough spoke by telephone Sunday from an acquaintance's home outside Sycamore.

DeKalb County State's Attorney Richard Schmack said in a filing last month that McCullough couldn't possibly have killed Maria. A review of old evidence and the discovery of some new evidence, he said, proved McCullough was 40 miles away at the time of Maria's abduction. Schmack, who became state's attorney only as McCullough's trial was ending, described the investigation and trial as deeply flawed.

McCullough said Sunday he thinks investigators wanted to be able to boast they had a hand in securing the conviction of one of the oldest cold cases in the country to ever go to trial.

Schmack said during the Friday hearing that ended with McCullough's release that his office will use its discretion and not retry McCullough. He said his office was fully convinced of McCullough's innocence.

Maria's disappearance made headlines nationwide in the 1950s.

She had been playing outside in the snow with a friend on Dec. 3, 1957, when a young man approached, introduced himself as "Johnny" and offered them piggyback rides. Maria's friend dashed home to grab mittens, and when she came back, Maria and the man were gone.

Forest hikers found her remains five months later.