Answering requests from media organisations lodged under the Freedom of Information Act, it released some 1,200 pages of documents about the crooner, spanning several decades.
The compendium, a 10in- thick doorstop of fading and heavily annotated papers, reveals a hodgepodge of claims and counter-claims about the singer, who died in May this year.
They range from allegations that Sinatra - whose nicknames included Ol' Blue Eyes and the Chairman of the Board - dodged the draft during the Second World War to details of death threats that were made against him.
There did not appear to be any single bombshell, however, that might stain the legend of Sinatra, who remains one of the foremost icons of American pop and entertainment.
For most of his life the New Jersey-born performer fought innuendo and rumour - and FBI investigation - arising from his involvement in the high- rolling casino world of Las Vegas and his contacts with the Mafia underworld.
Among Mafia figures whom Sinatra counted as his friends were Sam Giancana, Charles "Lucky" Luciano and Al Capone's cousin Joseph Fischetti. The ties between the singer and the Mafia first became headline news in 1947, when he attended a lavish party in Cuba in honour of Luciano, who had at that time been deported from the United States.
The FBI said it was releasing all but 25 pages of its files on Sinatra, dating all the way back to a 1938 mugshot taken when he was arrested on seduction charges. The crooner, agents said, had seen all the material himself after he had filed his own request for access to them in 1979 and 1980.
Among the papers yesterday was the disclosure that the FBI received tip- offs from the late entertainment columnist Walter Winchell. In 1944 Winchell told agents about alleged death threats to a baby of the singer. In the same year he said Sinatra had paid $40,000 for a government classification to avoid being drafted to serve in the war. The FBI investigated but found the charges to be baseless.
A memorandum dated 1971 and covered with pencil marks and underlinings claims that Sinatra was part of an alleged conspiracy to defraud $100,000 from a stockbroker named Ronald Alpert. Among others listed as part of the plot are several well-known former crime figures, including Aniello Dellacroce, Carlo Gambino, of the infamous New York Gambino Mob family, and Guiseppo "Joe" Gallo.
There are serial documents describing, in the dry language of law-enforcement agents, occasions when Sinatra had been the target both of death threats and extortion schemes. There are FBI memos, for example, on a report of a 1966 bomb threat against him in Miami Beach as well as a 1969 threat in which Sinatra was given the option of facing death or donating $2m to the Vatican. The sender of that letter was not prosecuted but dispatched by federal agents for psychiatric treatment.
Included in the files is a hand-written letter from a self-described psychic claiming that the singer was a political subversive bent on dividing the United States "West against East, East against West".
Sinatra also suffered from reports that he was a closet "red under the bed". One section of yesterday's compendium includes a 1955 message to the legendary director of the FBI, J Edgar Hoover, from the Philadelphia field office reporting that an informant had claimed "Frank Sinatra, well- known radio and movie star, is a member of the Communist Party". Another memo from a field agent in Detroit, however, says that Sinatra "was never active in the CP or related front-group activities in the state of Michigan".
Throughout his life Sinatra denied all the allegations that were made against him.
When his Cuba visit stirred suspicion of Mafia involvement, he famously replied: "Any report that I fraternised with goons and racketeers is a vicious lie." But he added: "I was brought up to shake a man's hand when I am introduced to him without first investigating his past."