While the FBI was investigating possible involvement of the Soviet Union in the 1963 assassination of President John F. Kennedy, the Soviet authorities were voicing suspicions that U.S. right-wing groups -- and even Kennedy's own vice president -- were behind the killing, newly released documents show.

The Soviet KGB claimed it had information tying Lyndon B. Johnson, who became president as a result of the assassination, to the killing, according to a 1966 letter to a presidential assistant from FBI Director J. Edgar Hoover that was released for the first time late on October 26.

The letter is among 2,800 previously classified Kennedy assassination documents that were released this week following an order by U.S. President Donald Trump. According to White House officials, Trump said in a memorandum that he had "no choice" but to keep some files secret because of national security concerns raised by the FBI and CIA.

The documents capture the frantic days after the November 22, 1963, assassination, during which federal agents madly chased after tips and sifted through leads worldwide.

But Kennedy scholars say the thousands of documents do not appear to contain any bombshell revelations about the killing that shocked the world.

The claim was contained in instructions from Moscow to the KGB residency in New York "to develop information" on Johnson, Hoover said in the letter, which cited an "FBI source" that had "furnished reliable information in the past."

Johnson has long been a focus of some conspiracy theorists, but no credible information has ever linked him to the assassination.

The documents show that the FBI's own chief suspect right after the assassination was the Soviet Union, with much attention given to assassin Lee Harvey Oswald's contact with "a member of the Soviet KGB Assassination Department" at the Soviet Embassy in Mexico, documents showed.

But Moscow believed Oswald was a "neurotic maniac" whose goal was to further a right-wing conspiracy trying to poison U.S.-Soviet relations, according to a just-released U.S. intelligence report issued days after the assassination.

Later, in May 1964, Soviet leader Nikita Khrushchev met influential Washington newspaper columnist Drew Pearson in Cairo, Egypt, and told him that he thought a right-wing conspiracy was behind the killing, according to another intelligence report.

Khrushchev told Pearson he could not believe the conclusion investigators had reached at that time: that both Oswald and Jack Ruby, the nightclub owner who fatally shot Oswald, had acted alone.

"He did not believe that the American security services were this inept," according to a CIA report of the discussion.

Pearson "got the impression that Chairman Khrushchev had some dark thoughts about the American right-wing being behind this conspiracy" and rejected all arguments to the contrary, the report said.

With reporting by AP and AFP