Hoffa was last seen July 30, 1975, and a second generation of authorities continues to work on the case, the Chicago Tribune reports.
Oakland, County (Mich.) Prosecutor David Gorcyca, who was 13 at the time of Hoffa's disappearance, admits, "We're not very optimistic due to the lapse of time and the deaths of a number of key players or witnesses."
But that doesn't mean investigators have given up. Gorcyca told the Tribune authorities still follow up on leads even if they have led to "laughable conclusions."
Police have searched for Hoffa's body in the New York Giants football stadium, swamps and incinerators. They have dug up a swimming pool and ripped up the floor of a house in Detroit. But there's been no sign of the missing union leader.
Gorcyca told the Tribune, "Unless we get a deathbed confession, it might be one of the greatest whodunit mysteries ever."