On the morning of July 31, 1975, James Riddle Hoffa, one of the most influential American labor leaders of the 20th century, is officially reported missing after he failed to return home the previous night. Though he is popularly believed to have been the victim of a Mafia hit, conclusive evidence was never found and Hoffa’s fate remains a mystery.

READ MORE: What Happened to Jimmy Hoffa?

Born in 1913 to a poor coal miner in Brazil, Indiana, Jimmy Hoffa proved a natural leader in his youth. At the age of 20, he helped organize a labor strike in Detroit, and remained an advocate for downtrodden workers for the rest of his life. Hoffa’s charisma and talents as a local organizer quickly got him noticed by the Teamsters and carried him upward through its ranks. Then a small but rapidly growing union, the Teamsters organized truckers across the country, and through the use of strikes, boycotts and some more powerful though less legal methods of protest, won contract demands on behalf of workers.

Hoffa became president of the Teamsters in 1957, when its former leader was imprisoned for bribery. As chief, Hoffa was lauded for his tireless work to expand the union, and for his unflagging devotion to even the organization’s least powerful members. His caring and approachability were captured in one of the more well-known quotes attributed to him: “You got a problem? Call me. Just pick up the phone.”

Hoffa’s dedication to the worker and his electrifying public speeches made him wildly popular, both among his fellow workers and the politicians and businessmen with whom he negotiated. Yet, for all the battles he fought and won on behalf of American drivers, he also had a dark side. In Hoffa’s time, many Teamster leaders partnered with the Mafia in racketeering, extortion and embezzlement. Hoffa himself had relationships with high-ranking mobsters, and was the target of several government investigations throughout the 1960s. In 1967, he was convicted of bribery and sentenced to 13 years in prison.

While in jail, Hoffa never ceded his office, and when Richard Nixon commuted his sentence in 1971, he was poised to make a comeback. Released on condition of not participating in union activities for 10 years, Hoffa was planning to fight the restriction in court when he disappeared on the afternoon of July 30, 1975, from the parking lot of a restaurant in Detroit, not far from where he got his start as a labor organizer. His family filed a missing persons report to the Bloomfield Township police the next day. Several conspiracy theories have been floated about Hoffa’s disappearance and the location of his remains, but the truth remains unknown.

READ MORE: 6 Mysterious Disappearances in US History