This might be the coldest case ever solved. Seven-year-old Maria Ridulph, was last seen alive in Sycamore, Illinois, in December 1957 with a blond-haired young man named Johnny who had approached her and a friend. In the spring of 1958, her body was found 120 miles (193 kilometers) away [source: O'Neill].
The case happened so long ago that the FBI director who demanded daily updates on the investigation was J. Edgar Hoover. Agents soon zeroed in on a suspect, a teenager named John Tessier, but they were unable to refute his alibi that he had taken a train from Chicago to Rockford, Illinois, with a ticket given him by a military recruiter on the evening of Riduph's disappearance. A polygraph test didn't indicate deception, so they scratched him off the list.
But then, roughly a half-century later, Tessier's sister Janet contacted the Illinois state police and revealed a deep dark secret: Her mother, on her deathbed in 1994, had blurted out that John Tessier was indeed the killer. Investigators reopened the case against Tessier, by then in his 70s, who long ago had moved away and changed his name to Jack McCullough.
Then they got another big break. The suspect's high-school girlfriend had an old picture of him, and tucked in the frame, detectives found an unused train ticket from the night of the murder. That demolished his old alibi. At trial, his 1957 photo was identified by the victim's friend, who inexplicably had not been shown his picture years before. In 2012, McCullough was sentenced to life in prison [sources: Goode, O'Neill, Schott].