The FBI said at the time that the disappearance could have been linked to Hoffa's efforts to regain power in the Teamsters and to the mob's influence over the union's pension funds.

Police and the FBI have searched for Hoffa intermittently ever since.

In September 2001, the FBI found DNA that linked Hoffa to a car that agents suspected was used in his disappearance.

In 2004, authorities removed floorboards from a Detroit home to look for traces of blood, as former Teamsters official Frank Sheeran claimed in a biography that he had shot Hoffa. Sheeran died in 2003.

Investigators ruled blood found in the house was not Hoffa's. The FBI has a sample of his DNA from a hair brush.

Two years later, the FBI razed a horse barn in Michigan following what it called "a fairly credible lead."

But the disappearance remains unsolved.

Read more: FBI sources: Hoffa buried in Roseville story not adding up

Urban lore long suggested that Hoffa was buried around the end zone at the former Giants Stadium in New Jersey.

As TruTV puts it, the mystery surrounding Hoffa is not simply a "whodunnit."

Jimmy Hoffa 2 "The likely suspects are all known, and their motives are well documented. The question is: Where? What exactly did they do to Jimmy Hoffa, and where did they dispose of his body?"

Read: Jimmy Hoffa's daughter says doubts her father's body will be found in Roseville

But over the years, numerous theories have been floated. In 1987, Joe Franco -- a former Hoffa strong-arm -- and a New York Times reporter published "Hoffa's Man," which Fortune described as "the hair-raising inside story of Jimmy Hoffa."

"Rather than being kidnapped by rival union forces as law enforcement authorities have long speculated, Franco says Hoffa was abducted by two federal agents," Fortune reported. "He thinks they drove Hoffa to a nearby airport, took off in a small plane, and pushed him out over one of the Great Lakes. Franco says he did not tell federal investigators this bizarre, and unverifiable, story because they would not grant him immunity."

Hoffa's son, James P. Hoffa, is the current president of the Teamsters.

Roseville police at Hoffa dig house 1

-- Roseville police have been at the home in anticipation for Friday's dig.