Lee Harvey Oswald met a senior KGB agent months before the assassination of John F Kennedy, documents from the newly-released trove have revealed.
Oswald met agent Valeriy Vladimirovich Kostikov in Mexico City's Soviet embassy two months before the president was shot and killed in Dallas on November 22, 1963, the Daily Mail reported.
Kostikov was a senior agent in the 13th Department which was responsible for assassinations.
Another document reveals the FBI was aware of the meeting on October 1 of 1963, more than a month before JFK was killed, meaning Oswald was already on their radar.
The trove of files were published on the National Archives website Thursday night (US time), but President Donald Trump has kept some back so federal agencies can black out portions.
In the documents FBI agents were grilled by a Senate committee about failing to stop Oswald's visit to Mexico in September, two months before JFK was killed on November 22 of 1963.
An FBI agent said: "Oswald wrote me in early 1962 to help expedite an exit visa for his wife. Why in the world would he tell a plopper like that?"
A Senator replies: "In any event, he told what the agent knew was lies, and what I am trying to get at is there was no analysis within the bureau of any of this.
"He even goes to Mexico City, contacts the Cuban consulate and the Soviet embassy, happens to be in contact - we don't know if there is anything sinister about it - with an agent who is known to be KGB by the FBI and by the CIA, and suspected of being Department 13, which is their assassination and sabotage squad.
"In any event, he then returns to the United States, is never again interviewed by the FBI."
A memorandum dated July 1969, titled "An investigation of Lee Harvey Oswald in Mexico", reports that the infamous assassin was seen at a party in Mexico in late September, 1963.
The CIA document reveals how Oswald went to a party in Mexico, along with two other Americans and hosted by Mexican playwright Elena Garro de Paz.
De Paz was interviewed by the FBI following the assassination who told the FBI about the other Americans with Oswald, but that the "interrogation was entirely unsatisfactory by normal investigatory standards."
It was in Mexico that Oswald, who was a communist, attended the Soviet Embassy in Mexico City on September 28 during the course of his six-day trip.
The FBI had discounted de Paz's accounts, but the CIA was "disturbed" by them.
In yet another shocking turn in the events surrounding the assassination and the follow up investigation by the FBI, the documents reveal the FBI's failure to properly investigate de Paz's story could have "damaged" the Warren Report.
The Warren Report, or Warren Commission was the government's investigation into Oswald and the assassination, and it concluded that he acted alone.
The report says "the credibility of the Warren Report would be damaged all the more if it were learned that these allegations were known and never adequately investigated by the competent American authorities".
It has been reported that Oswald was a good shot, but the speculation mounts that he may not have acted alone as more is revealed about his trip to Mexico.
A British newspaper reporter received an anonymous call 25 minutes before the JFK assassination tipping him to call the US Embassy "for some big news", it has been revealed.
The strange call to the Cambridge News on November 22, 1963 was recounted in a memo from the CIA to the FBI, made public in the Thursday release of files related to the John F. Kennedy assassination.
The memo is based on a CIA Station London cable just the morning after the assassination.
The senior reporter at the newspaper in Cambridge, England said he received an anonymous telephone call at 6.05pm GMT on November 22, urging him to "call the American Embassy in London for some big news".
The newsman reported the strange call to authorities, and MI-5, the UK's domestic security service, determined that the call had taken place 25 minutes before the US president was shot an ocean away in Dallas.
MI-5 advised their CIA counterparts that the reporter was "known to them as a sound and loyal person with no security record".
The British security service noted that the coincidental timing of the call fit the pattern of similar calls made across the UK in connection with the "case of Dr. Ward".
This is an apparent reference to Stephen Ward, an osteopath at the centre of the 1963 Profumo affair, a British political scandal.
CIA Deputy Director James Angleton sent the memo to the FBI Director on November 26, 1963.
Another document says the CIA's "plan in passing information to the Warren Commission was to eliminate mention of wire taps in order to protect their continuing operations" suggesting the spy agency withheld material from the investigation.
The Soviet Union also feared they were going to get blamed for putting "neurotic maniac" Oswald up to assassinating JFK the files revealed.
In one document dated December 1, 1963, from director J Edgar Hoover, the agency reveals everything it has learned about the Soviet reaction to the JFK's assassination and what its agents might have known about Oswald.
The tranche of documents reveal that the Soviets were just as obsessed with conspiracy theories surrounding Kennedy's death as the rest of the world.
"According to our source, officials of the Communist Party of the Soviet Union believed there was some well-organised conspiracy on the part of the 'ultraright' in the United States to effect a 'coup'."
In yet another document it's revealed the Cuban ambassador to the UN and his staff reacted with "happy delight" just days after the assassination.
It was revealed during the assassination investigation that Oswald had connections to Fair Play for Cuba, a grassroots movement to support the Cuban revolution.
The Cuban ambassador was instructed to govern their staff, and to "cease looking happy in public".
Before the assassination of JFK it is being revealed in the enormous trove of files that the CIA had considered attempts on Castro's life that included contaminated skindiving suits and exploding seashells.
The idea was to dust the inside of Castor's suit with a fungus that would produce a disabling and chronic skin disease and contaminating the breathing apparatus with the bacterium that causes tuberculosis.
The man that was supposed to give him the tainted suit was New York Attorney James Donovan negotiating with Castro for the release of the Bay of Pigs prisoners, however he had already given him a suit that was not contaminated.
The booby-trapped seashell scheme was to have a shell that exploded when lifted, but that idea was also abandoned after the CIA concluded there was no shell large enough in the waters where Castro would dive to contain the desired explosives.
Thursday the White House released 2800 records relating to the assassination of JFK, but a during the course of Thursday it was announced they would be holding back portions of the data as a matter of "national security".
The officials speaking on behalf of President Donald Trump said he had "no choice" in keeping some under lock and key pending a six month review process.
Russia has previously denied any involvement in the assassination of JFK.
Russian Foreign Ministry Spokeswoman Maria Zakharova told reporters Thursday: "For decades, information has been kept under lock and key.
"If even here wild insinuations are hurled at Russia, that would be a shame, because it is information, and not disinformation that people want."
September 26, 1960: Kennedy appears in the first televised election debates in history as he campaigns to become leader. The debate was seen as the turning point in the campaign.
November 8, 1960: JFK defeats Republican Richard Nixon with 49.7 percent of the vote. Nixon had 49.5 percent.
January 20, 1961: Inauguration for the new leader.
March 1, 1961: Kennedy establishes the Peace Corps, a volunteer program to promote world peace.
September 12, 1962: Kennedy announces that the US want to put man on the moon
October 1962: Cuban Missile crisis which is regarded as the closest the world ever came to nuclear war. Tensions rose after the USSR placed nuclear weapons in Cuba within striking distance of the U.S. The crisis eventually passed without a shot being fired.
June 11, 1963: The President calls for civil rights legislation to be brought in.
August 5, 1963: The U.S. and the USSR sign a nuclear test ban treaty pledging not to test bombs in air, space and water.
November 22, 1963: The President was assassinated in Dallas, Texas. He was shot once in the back and then in the head.
Sixteen bodies were recovered from a nine-storey building in Solomyansky.