OAKLAND TOWNSHIP, Mich. - The excavation of a rural field in suburban Detroit has failed to turn up the remains of former Teamsters union leader Jimmy Hoffa, the FBI announced Wednesday, adding another unsuccessful chapter to a nearly 40-year-old mystery.

"We did not uncover any evidence relevant to the investigation on James Hoffa," said Robert Foley, head of the FBI in Detroit.

Authorities have pursued multiple leads as to Hoffa's whereabouts since his disappearance in 1975.