Sources at the scene said investigators are searching a concentrated area at the site. Tents are set up in the middle of the property, at least 500 feet off the dirt road.

The Oakland Township property came under scrutiny in January after Tony Zerilli, 85, the son of reputed former Detroit mob boss Joseph Zerilli told broadcast media that Hoffa, 62, was buried there.

John Anthony, who worked the case as an FBI agent and later served as an FBI spokesman, said then that Zerilli was in a position to know secrets, including the fate of the former president of the International Brotherhood of Teamsters.

Dan Moldea, author of the book "The Hoffa Wars," who has studied the case closely for decades, agreed.

"This isn't some screwball," Moldea said. "He's the right man at the right time. His father would have had to sign off on this. The question is: Would his father or his father's associates have confided in him? I find it very possible that they did."