FBI agents are searching a farm near the city of Detroit after what they describe as a lead on the disappearance of union boss Jimmy Hoffa in 1975.
The FBI is using heavy construction equipment at the farm
Hoffa, who led the powerful Teamsters union, disappeared in July that year - allegedly after meeting a mafia boss. His body has never been found.
He was last seen at a restaurant some 20 miles (32km) from the farm.
Rumours have persisted that Hoffa was murdered by the mafia to prevent him regaining control of the union.
Investigators believe Hoffa, who led the Teamsters from 1957 to 1967, may have been the victim of an underground feud.
The BBC's Jeremy Cooke in New York says this could be the final chapter in one of the US's longest running crime mysteries.
Federal agents searching at the farm said they were acting on a tip-off.
There are many rumours surrounding Hoffa's disappearance
"This is probably a fairly credible lead. You can gather that from the number of people out here," said Special Agent Daniel Roberts.
Nothing significant has been discovered so far, agents say, but excavation work at the site is expected to last up to two weeks.
The FBI is using cadaver dogs, archaeologists and heavy construction equipment.
Mr Roberts said a barn might have to be dismantled.
Hoffa is reported to have fallen out with several organised crime figures after he was convicted of jury tampering and fraud in the 1960s.
After his release from prison in 1971, and shortly before his disappearance, Hoffa had been trying to regain control of the Teamsters.
On the day he disappeared, he was supposed to be meeting reputed Mafia figure Anthony "Tony Jack" Giacalone.
Hoffa was last seen in the car park of the restaurant.
One notable rumour is that Hoffa's remains were buried in the foundations of the Giants football stadium near New York.