Lee Harvey Oswald is pictured with Dallas police officers on November 22, 1963. Oswald was killed on November 24.

The FBI was told that Lee Harvey Oswald was going to be killed the night before it happened, it has been revealed in files just released by the US government related to the assassination of former president John F. Kennedy.

The security services received a tip-off from someone speaking in a "calm voice" informing them that a clandestine committee had agreed to take out Oswald.

The message was passed on to Dallas police, who were holding the suspected killer at the time, but Oswald was still gunned down the next day by Jack Ruby, a nightclub owner.

Oswald was killed by Jack Ruby, a nightclub owner.

Oswald's killing, two days after Kennedy's assassination, meant he was not able to be fully interrogated - a fact that has been latched on to by conspiracy theorists.

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It is just one of a series of apparent blunders by US security services revealed in the latest tranche of JFK files to be released.

Another is that the FBI had been tracking Oswald, a known Communist sympathiser, one month before the attack but failed to apprehend him.

A secret memo by J. Edgar Hoover, the FBI director, on November 24, 1963 - the day Oswald died and two days after Kennedy's assassination - reveals the bureau had been tipped off.

Hoover wrote: "There is nothing further on the Oswald case except that he is dead. Last night we received a call in our Dallas office from a man talking in a calm voice and saying he was a member of a committee organised to kill Oswald.

"We at once notified the chief of police and he assured us Oswald would be given sufficient protection.

"This morning we called the chief of police again warning of the possibility of some effort against Oswald and again he assured us adequate protection would be given. However, this was not done."

Hoover called the failure "inexcusable" and said an agent was sent to Oswald's hospital "in the hope that he might make some kind of a confession before he died, but he did not do so".