"This has finally come to an end. It has been an arduous project to get to this point," Chasnick said. "Hoffa's body is somewhere in that field, no doubt about it."
Detroit FBI chief Robert Foley made no mention of Zerilli's claims, merely saying investigators had obtained a warrant to search the field in Oakland Township, 25 miles north of Detroit.
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. Chasnick said he is "intimately involved" with people who know where the body is buried.
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 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.
Andrew Arena, who was head of the FBI in Detroit until he retired in 2012, said Zerilli "would have been in a position to have been told" where Hoffa was buried.
"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."
Keith Corbett, a former federal prosecutor in Detroit who was active in Mafia prosecutions touching on the Hoffa case, said it was appropriate for the FBI to act on Zerilli's assertions.
"You have a witness who is in a position to know, who says he has specific information," Corbett said. "The bureau has left no stone unturned."
Corbett also defended authorities for repeatedly spending time on what turned out to be dead ends.
"Anytime you look for somebody and don't find the body, it is embarrassing," Corbett said. "The thing the public isn't aware of, but police know, is there are a lot of dead ends in an investigation."
Associated Press writer Ed White in Detroit contributed to this report.