Liston was the much-feared heavyweight champion going into the fight with a 22-year-old Clay on February 25, 1964 but it was the latter who declared himself 'King of the world' as Liston did not return for the eighth round.
Clay, who later changed his name after converting to Islam, went on to become one of the biggest global sporting superstars and leave lasting memories in the ring following tussles with Joe FrazIer and George Foreman, as well as a second win over Liston.
But now the Washington Times has gained access to files which show the FBI did investigate whether the fight was fixed, under the Freedom of Information Act.
There is no suggestion from the acquired memos that Ali was in any way involved but, with FBI director J Edgar Hoover the addressee of the documents, it is clear the suspicions were taken seriously, even if no charges were ever brought.
Ash Resnick, a Las Vegas gambler, was the main suspect and it is a memo of an interview with fellow gambler Barnett Magids that is the most suggestive of foul play.
"At about noon on the day of the fight, (Magids) reached Resnick again by phone," the memo reads.
"At this time, Resnick said for him to not make any bets (on the Clay-Liston fight), but just go watch the fight on pay TV and he would know why and that he could not talk further at that time.
"Magids did go see the fight on TV and immediately realised that Resnick knew that Liston was going to lose," the document states.
"Later people 'in the know' in Las Vegas told Magids that Resnick and Liston both reportedly made over one million dollars betting against Liston on the fight."
Because of Clay's unexpected victory there were instantly questions as to the validity of the fight but Liston always maintained he went into the bout with an injured shoulder, the reason he did not return after the seventh round.
Liston - who died in 1970 - was allowed his share of the purse and, following a number of hearings, no evidence was found to suggest it was anything other than a clean fight.