Her disappearance and the subsequent massive search made national headlines, and it was said that President Dwight Eisenhower and FBI Director J. Edgar Hoover were asking for regular updates on the case. Maria's body was found in April 1958.
McCullough was one of more than 100 people who were briefly suspects, but he had what seemed like a solid alibi. On the day of the girl vanished, he told investigators, he'd been traveling to Chicago for a medical exam before joining the Air Force.
McCullough ultimately settled in Seattle and was a Washington state police officer.
Once a new investigation was launched, authorities went to Chapman and showed her an old photograph of McCullough. She told them the picture showed the teenager who came up to her and Maria that snowy day and identified himself as "Johnny."
After his conviction, McCullough wrote a letter addressed to Sycamore residents claiming FBI documents that he said backed his alibi had been barred from the trial.
The (DeKalb) Daily Chronicle reported that McCullough wrote: "If all parties had read the documents, it should have caused a reasonable person to conclude that I could not have been 'Johnny,' because at the exact time of the kidnapping, I was in Rockford, 40 miles away."
McCullough did not testify during his trial. He will have a chance to speak at Monday's sentencing.