Using the description of the car in which Hoffa was last seen by the lorry driver, FBI investigators were able to trace the maroon Mercury to its owner, Joe Giacalone, the son of mobster Anthony Giacalone. Giacalone claimed that on the day Hoffa was last seen, he had loaned the car to a friend, a teamster named Charles 'Chuckie' O'Brien, who was close to the Hoffa family and had actually lived with them at one time.
The car was soon located and O'Brien's fingerprints were found on a ‘7-UP’ soft drink bottle and a piece of paper recovered from the car. Investigators believed that Hoffa would have felt comfortable enough with O'Brien, whom he regarded as a son, to get into the Mercury without force.
The next step was for FBI agents to check on the whereabouts of the two men Hoffa was supposed to be meeting that day. 'Tony Jack' Giacalone claimed that he was at the gym, where he went every day. Witnesses confirmed they had seen him at the Southfield Athletic Club at the time of Hoffa's disappearance. 'Tony Pro' Provenzano said he has been in New Jersey playing cards with friends. Both suspects claimed they knew nothing about a scheduled meeting with Hoffa.
Chuckie O'Brien, who had apparently been driving the car in which Hoffa was last seen, claimed that he had delivered frozen salmon to the home of a Teamster International vice president and then helped the man's wife cut the fish into steaks on the morning of 30th July. While Hoffa had been waiting at the restaurant, O'Brien said he was also at the Southfield Athletic Club with Anthony Giacalone. O'Brien then said he had taken the loaned car to be washed because fish blood had leaked onto the back-seat. Unlike Giacalone's alibi, no one at the gym or the car wash could corroborate his story.
Specially trained German shepherds were flown in from Philadelphia eight days after Hoffa's disappearance. The dogs were given a pair of the labour leader's Bermuda shorts and a pair of his moccasins. They picked up Hoffa's scent in the back-seat and trunk of Joe Giacalone's maroon Mercury but with no body, there was nothing on which to base an arrest.
There have been many theories about what happened to Hoffa that fateful day in 1975. One theory was that Hoffa’s body was put into a 55-gallon steel drum and driven away in a truck. The drum was subsequently buried in the grounds of a toxic waste site in New Jersey. Another theory was that Hoffa’s body was mixed into the concrete that was used to construct the New York Giant’s football stadium in New Jersey. Some suggested that Hoffa was buried in a gravel pit in Michigan, which was owned by his brother William Hoffa. Perhaps one of the most gruesome theories was that Hoffa had been ground up at a meat processing plant and then dumped in a Florida swamp or disintegrated at a fat-rendering plant.