The murder of Lee Harvey Oswald could have been prevented after the FBI were warned of his impending death - but Dallas police failed to protect him, it has emerged.

Nearly 3,000 documents related to JFK's shooting have been declassified by The White House, among them remarks made by then-FBI boss J Edgar Hoover the day Oswald died.

According to Hoover, the FBI were contacted by a man who said a 'committee' was plotting to kill Oswald, who had been arrested for killing the president; the feds then told police in Dallas to protect the presidential assassin.

Instead, Oswald was walked out in front of a crowd, cameras - and local nightclub owner Jack Ruby, who marched out and shot him dead.

Scroll down for video

Newly released memos from J Edgar Hoover reveal that the FBI had been warned by a man with a 'calm' voice that a 'committee' was planning to kill Lee Harvey Oswald (pictured being shot)

Hoover said the feds told Dallas PD twice to protect Oswald, but the man was still gunned down by small-time crook Jack Ruby (pictured); Hoover said that was 'inexcusable'

Speaking on November 24, 1963, the morning Oswald was killed, Hoover said: '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 organized 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.

Hoover (pictured) also worried about the public believing in a conspiracy theory. The documents are some of almost 3,000 released by the White House this week

'However, this was not done.'

Oswald was being transferred to Dallas County Jail when Ruby, who claimed to have mob connections, fired the fatal shot.

Hoover - who headed the FBI and its predecessor, the Bureau of Investigation, for a total of 48 years - said that getting Oswald killed before he could be interviewed was 'inexcusable.'

'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,' he said.

'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.'

Hoover also expressed concerns about conspiracy theories forming around Kennedy's murder, just days after the death.

'The thing I am concerned about, and so is [deputy attorney general Nicholas] Katzenbach, is having something issued so that we can convince the public that Oswald is the real assassin,' he wrote.

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

Hoover's suggestion was ignored and instead, President Lyndon Johnson set up the Warren Commission to investigate the assassination the following week.

The notes also reveal that the Soviet Union also suspected a conspiracy organized by the right-wing, or possibly Lyndon Barnes Johnson, Kennedy's successor.

All of the documents are related to the killing of JFK (pictured) in 1963. They also say that the Soviets believed Kennedy was killed either by Lindon Barnes Johnson or right wingers

They certainly didn't believe that Oswald - a 'neurotic maniac who was disloyal to his own country and everything else' - was the killer, the notes claimed.

'Our source further stated that Soviet officials were fearful that without leadership, some irresponsible general in the United States might launch a missile at the Soviet Union,' a note says.

The documents also say that the Cuban ambassador to the US met the news of Kennedy's murder with a 'happy delight'.

It's not clear how Ruby might have got into the basement that Oswald was being transported through when he was shot.

But another memo released by the White House said that according to an FBI informant, Ruby had a 'good in' with the Dallas PD.

Ruby himself died of cancer in his prison cell four years later.