Authorities today were searching a field in Michigan for the body of former Teamsters boss Jimmy Hoffa, the latest in a decades-long string of searches for the missing labor leader who disappeared nearly 38 years ago.

Investigators started digging in a field in Oakland Township after a search warrant was issued in the case. The search area, according to ABC News Detroit affiliate WXYZ, is near land that was once owned by suspected mobsters. Approximately 20 federal agents focused their search Monday morning on one spot in the center of the field.

(Carlos Osorio/AP Photo)

In January, a reputed underboss of the Detroit mafia said he believed Hoffa was buried in the area. Tony Zerilli, who the FBI considers a key figure in the city's mafia, told reporter Marc Santia of WNBC in New York that Hoffa was going to be put "in a shallow grave" there and later taken upstate for "final burial" before the plan "fell through."

Zerilli, who was in jail at the time of Hoffa's disappearance, should be seen by authorities as a credible tipster, according to Dan Moldea, author of "The Hoffa Wars."

"Zerilli is a guy you have to take seriously," Moldea said in a phone interview Monday. "The question is: Does he really know something?"

At a news conference Monday afternoon, Zerilli's attorney, David Chasnick, said, "The Hoffa body is in that field - no doubt about it."

"There used to be a barn in that field," Chasnick said. "Buried under the barn, under the cement slab - that's where our understanding is that the body should be."

Chasnick said he believed the FBI had confirmed that the cement slab was still in the field.

Hoffa, who headed the powerful International Brotherhood of Teamsters Union at one point, vanished in July 1975. He was 62 at the time and had recently spent nearly five years behind bars.

In the past few decades, numerous tips have emerged about his whereabouts, only to lead to nothing. Last September, police in Roseville, Mich., about 20 miles north of Detroit, dug up a driveway. In 2009, FBI agents dug up a lumberyard in the city, prompting speculation that the excavation was a search for Hoffa. Three years earlier, in May 2006, a search for Hoffa at a farm in the town of Milford, Mich., was known as "The Big Dig."

Zerilli released a book, "Hoffa Found," earlier this year.