It was hailed as a victory for the persistence of justice. Some five and a half decades after a 7-year-old girl was kidnapped, strangled and stabbed in rural northern Illinois, Jack McCullough, a military veteran and former police officer, was convicted of the killing in 2012 in what was believed to be one of the oldest American cold cases to result in an arrest.

But now, the local prosecutor says that his office locked up the wrong man.

Richard Schmack, the state’s attorney for DeKalb County, Ill., about 60 miles west of Chicago, said on Friday that a review of evidence not presented at the trial has convinced him that Mr. McCullough, 76, could not have committed the crime. As such, Mr. Schmack, who was elected to his post after Mr. McCullough’s conviction, said he would not contest the defendant’s motion to have his conviction overturned at a hearing in county court on Tuesday.

Pending a judge’s review, Mr. McCullough could be released from prison.

“All the evidence I found pointed toward him being innocent,” Mr. Schmack said in an interview on Friday.

Mr. Schmack’s decision represented a remarkable turn in a case that shocked small-town Sycamore, and drew special attention from J. Edgar Hoover, then the director of the F.B.I., and President Dwight D. Eisenhower. The killing of the girl, Maria Ridulph, on Dec. 3, 1957, and the eventual conviction of Mr. McCullough inspired a book and a documentary. Maria’s brother, Charles Ridulph, said he was not surprised by Mr. Schmack’s decision because the prosecutor had been giving him regular updates during the investigation, and he seemed to be headed in that direction. But Mr. Ridulph, 70, said he was upset by the decision.