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 says 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 for several hours on Sunday, his second full day of freedom. On Friday, 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 for them to intervene. His letters were never answered, he said.

McCullough wasn't worried when he was first arrested in 2011 and his conscience was clear as he entered prison.

"I know I didn't do it," 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 by telephone from an acquaintance's home outside Sycamore. "I want the state to be (held) responsible."

DeKalb County State's Attorney Richard Schmack said in a filing last month that McCullough couldn't possibly have killed Maria. He said a review of old evidence and the discovery of some new evidence 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 that they had a hand in securing a conviction in one of the oldest cold cases in the country to go to trial.

Schmack said during the hearing Friday that led to McCullough's release that his office would 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.