Jimmy Hoffa (Associated Press)

James Riddle Hoffa hasn't been seen since July 31, 1975, when he was supposed to meet with some associates to try to iron out their differences. It was four years after Richard Nixon pardoned Hoffa, who had served five years in prison for jury tampering, conspiracy, and mail and wire fraud.

The former president of the Teamsters union – who had been accused of having close ties to organized crime figures – wanted to regain the job his arrest and conviction had cost him.

Hoffa was a charismatic union leader who had the loyalty of his membership, even with his dubious connections. They were connections he worked to advance the cause of making the Teamsters the most powerful union in the United States.

After Hoffa's 1964 conviction, he installed Frank Fitzsimmons as his successor – to hold his place until he got out of prison. While he was out of the union picture, Hoffa's associates grew comfortable with their relationship with Fitzsimmons, who was seen as much more pliant than Hoffa. The new Teamsters boss took a hands-off approach when it came to the union's relationships with certain independent businessmen.

As part of the pardon deal, Hoffa was prevented from holding union office until 1980. It's believed Hoffa was preparing an appeal of that ruling when he disappeared.

Who's been linked to Hoffa's disappearance?

A veritable who's who of organized crime, for the most part. Hoffa had been linked to underworld figures for decades. In 1959, Robert F. Kennedy told TV talk show host Jack Paar that the U.S. would not be controlled by people like Hoffa, Johnny Dio and Tony (Ducks) Corallo. Dio and Corallo were known gangsters. At the time Kennedy was chief counsel of the Senate labor rackets committee. He would become U.S. attorney general after his brother John F. Kennedy was elected president.

Although charges have never been laid in the case, investigators have had no shortage of suspects. Among them are:

Tony Provenzano

"Tony Pro" ran Local 560 of the Teamsters Union, based in Union City, N.J. Federal investigators believed he was a senior Mafia official who used the local to run his illegal operations. Provenzano was said to be a lifelong friend of Hoffa, but the two had a falling out after Hoffa was released from prison and once again became a force in the Teamsters. Federal investigators believe Hoffa was lured to the Red Fox restaurant in Bloomfield Hills, Mich., on the pretext of meeting with Provenzano to patch up their differences. In 1978, Provenzano was convicted of a 1961 murder and sent to prison, where he died 10 years later at the age of 81.

Anthony Giacalone

A reputed mob enforcer in Detroit, federal officials believe Giacalone set up the meeting between Hoffa and Provenzano. After Hoffa's disappearance, Giacalone was tried and convicted on tax evasion charges. He spent 10 years in prison. In 1996, he was charged with racketeering, but died before the case went to trial.

Charles O'Brien

O'Brien had a close relationship with Hoffa and was a longtime Teamsters associate. It's believed that O'Brien drove Hoffa to a house, where he was killed. O'Brien maintains that he spent part of the day that Hoffa disappeared cleaning a large fish that he had caught. He later moved to Florida but was eventually banished from the Teamsters for his mob connections. O'Brien has said it was the U.S. government and not the mob who killed Hoffa.

Salvatore Briguglio

Federal investigators believe Briguglio shot Hoffa. It's believed that Briguglio, his brother, Gabriel, and Thomas Andretta, a business agent with the Teamsters, waited in a house not far from the restaurant, for Hoffa to be delivered for execution. Salvatore Briguglio was killed – gangland style – in New York in 1978. It's believed he was on the verge of making a deal to testify against Provenzano in another murder.

Russell Bufalino

Bufalino was a northeastern Pennsylvania crime boss. Federal investigators believe he ordered the killing of Hoffa to prevent him from regaining the presidency of the Teamsters union. There were concerns that Hoffa was getting "too cozy" with federal officials and was ready to provide information on mob ties. Bufalino died in a nursing home in 1994 at the age of 90.

Frank Sheeran

The FBI focused its investigation on Frank Sheeran almost immediately after Hoffa's disappearance. Sheeran had been president of Teamsters Local 326 in Delaware and was said to be a close associate of Bufalino. He was known to have driven Bufalino to Detroit just before Hoffa disappeared.

Before his death, Sheeran told American writer Charles Brandt that he was involved in the murder of Hoffa. Sheeran told Brandt he was supposed to be at the July 31, 1975, meeting to provide backup for Hoffa. Brandt says Sheeran showed up in the car driven by O'Brien and said the meeting location had been changed. They drove off.

"When they got to the house … Sheeran and Hoffa got out of the car. Hoffa walked up the steps to the house with Sheeran behind him," Brandt told CBC Radio's As It Happens in a 2004 interview.

"When they got inside the house, Sheeran shot him in the back of the head twice."

Sheeran had repeatedly told federal investigators that Hoffa's murder was a government conspiracy.

Workers dig near a barn at a horse farm in Milford Township, Mich., where FBI agents, acting on a tip, were investigating Jimmy Hoffa's 1975 disappearance. (Paul Sancya/Associated Press)

So where'd they stash the body?

The rumours, they do abound – although most of them centre on sites near Detroit. Perhaps the rumour that attracted the most attention was that Hoffa's body was buried in the football stadium that is home to the New York Giants in East Rutherford, N.J. The complex – which also includes the arena where the New Jersey Devils play their NHL games – is known as the Meadowlands.

One version of that rumour said that Hoffa was buried in the end zone. Another suggested his body was encased in the stadium's concrete foundation. In 2004, an American television program – Myth Busters – used ground-penetrating radar to scan sections of the stadium for human remains. Nothing unusual was detected.

• A 40-hectare gravel pit in Highland, Mich., which was owned by his brother William.
• The foundation of a public works garage in Cadillac, Mich., where the body was encased in concrete.
• Underneath a swimming pool behind a mansion in Bloomfield Hills, Mich.
• Under the helipad at the Sheraton Savannah Resort Hotel. The hotel was owned by the Teamsters when Hoffa disappeared.
• The bottoms of lakes Erie, Huron and St. Clair.
• A toxic waste site in Jersey City, N.J., after Hoffa's body was placed in a steel drum.
• Under the New Jersey Turnpike.
• Under the Pulaski Skyway in a landfill site in Jersey City.
• Within the compacted remains of cars, after being crushed in an automobile compactor at Central Sanitation Services in Hamtrack, Mich.
• The bottom of Michigan's Au Sable River, after the body was weighted down and dumped.
• A field in Waterford Township, Mich., not far from the restaurant where Hoffa was last seen alive.
• Near New York City's Varian Narrows Bridge.
• A Florida swamp, where his remains were dumped after they were ground up at a meat-processing plant.
• A fat-rendering plant, where Hoffa's body was dissolved in a vat.

Another theory, put forth by a member of the Teamsters during questioning by investigators, was that Hoffa was not killed, but that he skipped the country with a "go-go" dancer and settled in Brazil – the South American country, not the Indiana town where he was born.

On May 16, 2006, investigators began digging at a horse farm about 50 kilometres west of Detroit. They said they were searching for "evidence of criminal activity that may have occurred when the properties were under previous ownership."