After leaving the New Orleans Police Department, Banister established his own private detective agency, Guy Banister Associates, Inc. at 434 Balter Building. In June 1960, Banister moved his office to 531 Lafayette Street on the ground floor of the Newman Building. Around the corner but located in the same building, with a different entrance, was the address 544 Camp Street, which would later be found stamped on Fair Play for Cuba Committee leaflets distributed by Lee Harvey Oswald, the suspected assassin of President John F. Kennedy. The Newman Building housed militant anti-Castro groups, including the Cuban Revolutionary Council (October 1961 to February 1962), as well as Sergio Arcacha Smith's Crusade to Free Cuba Committee. Banister's office was within walking distance of the New Orleans offices of the FBI, CIA, Office of Naval Intelligence and the Reily Coffee Company (Lee Harvey Oswald's employer and a supporter of anti-Castro Cubans). Banister was implicated in a 1961 raid on a munitions depot in Houma, Louisiana, in which "various weapons, grenades and ammunition were stolen ... which were reportedly seen stacked in Banister's back room by several witnesses." The "New Orleans States-Item" newspaper reported an allegation that Banister served as a munitions supplier for the 1961 Bay of Pigs Invasion and continued to deal weapons from his office until 1963. In 1962, Banister allegedly dispatched an associate, Maurice Brooks Gatlin — legal counsel of Banister's "Anti-Communist League of the Caribbean" — to Paris to deliver a suitcase containing $200,000 for the French OAS. In 1963, Banister and anti-Castro activist David Ferrie began working for a lawyer named G. Wray Gill and his client, New Orleans Mafia boss Carlos Marcello. This involved attempts to block Marcello's deportation to Guatemala. In early 1962, Banister assisted David Ferrie in a dispute with Eastern Airlines regarding charges brought against Ferrie by the airline and New Orleans police of "crimes against nature and extortion." During this period, Ferrie was frequently seen at Banister's office. Banister served as a character witness for Ferrie at his airline pilot's grievance board hearing in the summer of 1963. On the afternoon of November 22, 1963, the day that President John F. Kennedy was assassinated, Banister and one of his investigators, Jack Martin, were drinking together at the Katzenjammer Bar, located next door to 544 Camp Street in New Orleans. On their return to Banister's office, the two men got into a dispute.