It was the coldest of cold cases — a crime so old that when the director of the F.B.I. requested regular updates on the investigation, it was J. Edgar Hoover who did the asking.

But on Monday, 55 years after the 7-year-old Maria Ridulph disappeared from a street corner in Sycamore, Ill., an Illinois judge handed down a sentence for her kidnapping and murder, consigning Jack McCullough, 73, to serve the rest of his “natural life” in prison.

Mr. McCullough, a resident of Seattle and former police officer who has steadily maintained his innocence, was convicted of the murder in September. He has been held in the DeKalb County jail since his arrest in July 2011. The prison term, imposed under 1957 sentencing rules, carries the possibility of parole after 20 years.

Victor Escarcida, an assistant state’s attorney who helped prosecute Mr. McCullough, said he was pleased by the outcome.