Officials are "here to execute a search warrant, based on information that we have involving the disappearance of Jimmy Hoffa," Foley said.
He said the warrant is sealed and that details about what was sought would not be released. Foley did not take questions from reporters. Oakland County Sheriff Mike Bouchard, who joined Foley during a press conference, said it was his "fondest hope" to bring closure for Hoffa's family and the community.
"This has been one of those kind of open wounds for a long time," Bouchard said.
FBI agents and Oakland County sheriff's deputies worked in the field as the beep of excavating equipment moving in reverse and crickets hidden in the tall grass and weeds could be heard over nearby traffic. The field is surrounded by trees and dirt on three sides and a gravel road runs alongside the fourth.
Hoffa, Teamsters president from 1957-71, was an acquaintance of mobsters and an adversary of federal officials. The day in 1975 when he disappeared from a Detroit-area restaurant, he was supposed to be meeting with a New Jersey Teamsters boss and a Detroit Mafia captain.
Since then, multiple leads to his supposed remains have turned out to be red herrings.
In September, police took soil from a suburban backyard after a tip Hoffa had been buried there. It was just one of many fruitless searches. Previous tips led police to a horse farm northwest of Detroit in 2006, a Detroit home in 2004 and a backyard pool two hours north of the city in 2003.
In February, reputed Mafia captain Tony Zerilli told Detroit TV station WDIV in February that he knew where Hoffa was buried and that the FBI had enough information for a search warrant to dig at the site. He said he answered every question from agents and prosecutors, and had been promoting a book, "Hoffa Found."
Foley did not mention Zerilli's claims in his brief comments Monday.
Zerilli was convicted of organized crime and was in prison when Hoffa disappeared. But he told New York TV station WNBC in January that he was informed about Hoffa's whereabouts after his release.
Andrew Arena, who was head of the FBI in Detroit until retiring in 2012, was near the scene of the search on Monday. He said he has no direct knowledge of the search, "but it's pretty clear that's what they're doing."
"According to Zerilli and his description of the property, this seems to be it," Arena said.
Although Zerilli was in prison when Hoffa disappeared, later "he would have been in a position to have been told," Arena said.
"I still don't know if this was a guess on his part. I don't know if he was actually brought here by the Detroit (mob) family," Arena said. "It's his position as the reputed underboss. That's the significance."