Zerilli was in prison himself when Hoffa disappeared.
Zerilli, according to the law enforcement source, said that when he was freed, he asked a mob enforcer what happened to Hoffa.
The mobster allegedly told Zerilli that Detroit's crime bosses ordered the Hoffa hit. They lured him to a meeting and then drove him to a farm owned by a mob underboss. The enforcer allegedly told Zerilli that Hoffa was killed and buried on the property, which covers several acres.
The area being searched was described as relatively small, about the size of a small party tent, according to the source. Aerial video showed a somewhat larger area had been cleared of grass.
Zerilli has been to the site more than once, said Chasnick, who declined to elaborate.
Hoffa's daughter, Barbara Crancer, told CNN by phone Tuesday that she is always appreciative when the FBI follows credible leads in the case.
But, she said, she doesn't want to get her hopes up.
"We've been through it too many times," she said.
FBI Special Agent Bob Foley, head of the agency's Detroit office, told CNN at the scene that the information leading to the search "reached the threshold of probable cause, which was sufficient to allow us to obtain a search warrant." The paperwork supporting the search warrant is under seal.
"If it didn't rise to that level then, certainly, we wouldn't be out here," Foley said.
Hoffa's disappearance and presumed death have vexed investigators. As recently as October, soil samples were taken from a home in a suburban Detroit community after a tipster claimed he saw a body buried in the yard a day after Hoffa disappeared.
The soil samples were tested, and showed no evidence of human remains or decomposition.
Zerilli was freed in 2008 after his last prison sentence. Keith Corbett, a former U.S. attorney, told CNN earlier this year that Zerilli headed a Detroit organized crime family from 1970 to 1975, but was in prison when Hoffa vanished.