In the aftermath of President John F Kennedy’s assassination, the USSR was worried it would be blamed for putting Lee Harvey Oswald up to the attack, and that America could lash out in a state of leaderless panic.

Previously classified intelligence documents depict turmoil on both sides of the Atlantic, as officials attempted to piece together what had happened and exactly who was to blame.

Almost 3,000 newly released documents detail everything the US knew about the Russian reaction to the 1963 assassination and reveal the extent to which the Kremlin believed in conspiracy theories surrounding Kennedy’s death.

Russian sources reportedly described Oswald as a “neurotic maniac who was disloyal to his own country and everything else”.

Oswald was a former Marine, a self-proclaimed Marxist and repeatedly tried to defect to the USSR, but following a trip to Russia, he was not granted further access to the Soviet bloc.

Immediately after Kennedy’s assassination, given Oswald’s history, the USSR considered him a major danger to stability between the two countries, which had improved during Kennedy’s administration.

One source told US intelligence officials: “Soviet officials were fearful that without leadership, some irresponsible general in the United States might launch a missile at the Soviet Union.”

They also apparently believed he was part of a wider plot to kill the president, launched by far-right groups in the US.

According to the documents, “officials of the Communist Party of the Soviet Union believed there was some well-organised conspiracy on the part of the “ultra-right” of the United States to effect a “coup”. They seemed convinced that the assassination was not the deed of one man, but that it arose out of a carefully planned campaign in which several people played a part.”

The document adds: “They felt that those elements interested in utilising the assassination and playing on anti-communist sentiments in the United States would then utilise this act to stop negotiations with the Soviet Union, attack Cuba and thereafter spread the war. As a result of these feelings, the Soviet Union immediately went into a state of national alert.”

Later on, the files reveal the Kremlin’s first interaction with Oswald, during an earlier trip to the Soviet Union.

The documents cite a Soviet defector to the US, named Yuri I Nosenko, who told intelligence officials: “Oswald came to the attention of the KGB when he expressed a wish to defect to the Union of Soviet Socialist Republics shortly after his arrival in Russia. However the KGB, after enquiry, decided he was mentally unstable and informed him he had to return to the United States upon completion of his visit. Thereafter, when Oswald missed a sight-seeing tour he was to take, his hotel room was forced open and he was found with one of his wrists badly cut.”

The assassination of JFK – in pictures Show all 8 1 /8 The assassination of JFK – in pictures The assassination of JFK – in pictures President John F. Kennedy and his wife Jacqueline Kennedy ride with secret agents in an open car motorcade shortly before the assassination, 22 November 1963 Getty The assassination of JFK – in pictures President John F. Kennedy and his wife Jacqueline Kennedy prior to his assassination Keystone/Getty The assassination of JFK – in pictures Kennedy is struck by an assassin's bullet as he travels through Dallas in a motorcade In the car next to him is his wife Jacqueline and in the front seat is Texas governor John Connally Three Lions/Hulton Archive/Getty The assassination of JFK – in pictures The view from the sixth floor window of the Texas School Book Depository in Dallas, from which Lee Harvey Oswald is thought to have assassinated Kennedy. This photograph was taken approximately one hour after the assassination Hulton Archive/Getty The assassination of JFK – in pictures Lee Harvey Oswald during a press conference after his arrest in Dallas. Lee Harvey Oswald was killed by Jack Ruby on 24 November on the eve of Kennedy's burial AFP/Getty The assassination of JFK – in pictures Lyndon B. Johnson takes the oath of office as President of the United States on the day of Kennedy's assassination. Jackie Kennedy is stood next to Johnson Getty The assassination of JFK – in pictures Kennedy's funeral procession goes into Arlington Cemetery in Washington Newsmakers/Getty The assassination of JFK – in pictures Kennedy's casket sat in the East Room of the White House Newsmakers/Getty