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The highly-anticipated release of long-secret documents detailing the investigation into the 1963 assassination of President John F. Kennedy have provided fresh fodder to fuel conspiracy theories surrounding the controversial death of the former commander in chief.

President Trump on Thursday released the trove of records, however, the collection was incomplete, with some records being held back. Trump cited “potentially irreversible harm” to national security if he were to allow all records to come out now. He placed the remaining files under a six-month review, but released 2,891 others, racing to honor a deadline mandating their release.

On Nov. 22, 1963, Kennedy was shot and killed in Dallas by, authorities contend, a lone gunman, Lee Harvey Oswald.

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John F. Kennedy waves from his car during the motorcade in Dallas before he was shot and killed. (AP)

But a segment of the public never bought into the official explanation of Kennedy's assassination, citing video clips, interviews and science experiments in an attempt to prove Oswald, who was himself assassinated two days after Kennedy, did not act alone.

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The files released Thursday show the aftermath of Kennedy’s death included then FBI Director J. Edgar Hoover venting his frustration about Oswald's killing at the hands of Jack Ruby, bellowing that the shooting would cause the public to suspect a conspiracy, NBC News reported.

"There is nothing further on the Oswald case except that he is dead,” Hoover said. "The thing I am concerned about, and so is [deputy attorney general] Mr. Katzenbach, is having something issued so we can convince the public that Oswald is the real assassin."

Hoover reported the FBI received a call from the Dallas office stating the caller was part of a cabal “organized to kill Oswald.”

Hoover said he relayed that warning to Dallas police and was assured Oswald would be protected. However, Oswald was shot dead the next day by Jack Ruby.

“Oswald having been killed today after our warnings to the Dallas Police Department was inexcusable," Hoover said. "It will allow, I am afraid, a lot of civil rights people to raise a lot of hell because he was handcuffed and had no weapon. There are bound to be some elements of our society who will holler their heads off that his civil rights were violated — which they were."

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Authorities say Lee Harvey Oswald was the lone gunman responsible for John F. Kennedy's death. (AP)

Hoover suggested that “instead of a Presidential Commission, we can do it with a Justice Department report based on an FBI report.”

Hoover’s suggestion went nowhere. President Lyndon B. Johnson established the Warren Commission to investigate the assassination the following week.

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In 1964, the Warren Commission concluded Oswald and Ruby acted alone in their assasinations. No other blame was put on other organizations, actors or foreign governments.

Johnson was mentioned in the files as well. The Soviet Union believed Kennedy's vice president was behind the assassination, the New York Post reported. The Societs also feared they would be blamed for the slaying and attacked in retaliation.

In a Dec. 1, 1966 FBI memo, sources said the KGB, the world’s largest “spy and state-security machine” claimed they had “possession of data purporting to indicate President Johnson was responsible for the assassination of the late President John F. Kennedy.”

The memo, titled: “REACTION OF SOVIET AND COMMUNIST PARTY OFFICIALS TO JFK ASSASSINATION” was sent to Hoover.

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The files on President John F. Kennedy's killing were released on Oct 26, 2017. (AP)

“KGB headquarters indicated that in view of this information, it was necessary for the Soviet Government to know the existing personal relationship between President Johnson and the Kennedy family, particularly that between President Johnson and Robert and ‘Ted’ Kennedy,” the document stated.

The Soviet Union also feared the U.S. would use the assassination to bolster “anti-Soviet sentiment and even lead to an attack.”

Oswald lived in the Soviet Union for three years and married a woman from there. He reportedly had ties to the KGB while in America.

Johnson also believed in conspiracy theories himself, the New York Post reported. Richard Helms, the CIA director under Johnson, said the former president said he believed Kennedy was killed in retaliation for the assassination of South Vietnam President Ngo Dinh Diem.

Diem was arrested and killed by a CIA-backed coup in Vietnam in 1963.

The files also showed that a British reporter with the Cambridge News received an anonymous phone call prior to the assassination, alerting the reporter to “some big news.”

“The caller said only that the Cambridge News reporter should call the American Embassy in London for some big news and then hung up,” reads the document from former CIA Deputy Director James Angleton.

The reporter, who was not identified in the Nov. 26, 1963, report, “never received a call of this kind before and MI5 said that he is known to them as a sound and loyal person with no security record.” (MI5 is Britain's Security Service, similar to the CIA in the United States.)

After Kennedy’s death, the reporter told Cambridge police about the call and they informed M15.

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A British reporter said he received a phone call promising 'big news' prior to the assassination. (AP)

Despite the new details unleashed by the files, some felt the bath did not have the “smoking gun” many have urged for following the mysterious death. The full record will still be kept from the public for at least six months – and longer if agencies make a persuasive enough case for continued secrecy.

The Associated Press contributed to this report.