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The detailed three-page report implicates the Hollywood actor Peter Lawford, Monroe's psychiatrist, staff and her publicist in the plot.

The allegations suggest the 36-year-old actress, who had a history of staging attention-seeking suicide attempts, was deliberately given the means to fake another suicide on August 4, 1962. But this time, it is suggested, she was allowed to die as she sought help.

The document, hidden among thousands of pages released under freedom-of-information laws last October, was received by the FBI on October 19, 1964 - two years after her death - and titled simply "ROBERT F KENNEDY".

It was compiled by an unnamed former special agent working for the then Democrat governor of California, Pat Brown, and forwarded to Washington by Curtis Lynum, then head of the San Francisco FBI. Despite a disclaimer that it could not be sourced or authenticated, it was considered important enough to immediately circulate to the FBI's five most senior officers, including director J. Edgar Hoover's right-hand man, Clyde Tolson.

The report was in effect buried for decades as a classified document, and even the released version contains censored sections. Never before mentioned despite thousands of articles, books and documentaries about her death, it details aspects of Kennedy's on-and-off affair with the movie star, including sex parties and a lesbian dalliance, as well as her emotional departure from 20th Century Fox and descent into depression.

Critically, it raises an alleged conspiracy, apparently overseen by Lawford, for Monroe to unwittingly commit suicide with the drug Seconal, a barbiturate used to treat insomnia and relieve anxiety. The document gives no precise reason why she would be killed but hints it may be linked to her threats to make public her affair with Kennedy, as other conspiracy theories have previously claimed. It states in part: "Peter Lawford, [censored words blacked out] knew from Marilyn's friends that she often made suicide attempts and that she was inclined to fake a suicide attempt in order to arouse sympathy.

"Lawford is reported as having made 'special arrangements' with Marilyn's psychiatrist, Dr Ralph Greenson, from Beverley Hills. The psychiatrist was treating Marilyn for emotional problems and getting her off the use of barbiturates. On her last visit to him he prescribed Seconal tablets and gave her a prescription for 60 of them, which was unusual in quantity especially since he saw her frequently. On the date of her death … her housekeeper put the bottle of pills on the night table. It is reported that the housekeeper and Marilyn's personal secretary and press agent, Pat Newcomb, were co-operating in the plan to induce suicide."

It goes on to say that on the same day, Kennedy had booked out of the Beverley Hills Hotel and flown to San Francisco where he booked into the St Charles Hotel, owned by a friend. "Robert Kennedy made a telephone call from St Charles Hotel, San Francisco, to Peter Lawford to find out if Marilyn was dead yet."

Lawford called and spoke to Monroe "then checked again later to make sure she did not answer". The document claims the housekeeper, Eunice Murray, who had been hired by the actress on the advice of Dr Greenson, then called the psychiatrist.

"Marilyn expected to have her stomach pumped out and get sympathy for her suicide attempt. The psychiatrist left word for Marilyn to take a drive in the fresh air but did not come to see her until after she was known to be dead."

Officially, the actress was found by Murray in the early hours of August 5, naked on her bed lying on top of her telephone. The others are now dead, too.

The FBI report says Kennedy had promised Monroe he would divorce his wife and marry her, but the actress eventually realised he had no intention of doing so. About this time, he had told her not to worry about 20th Century Fox cancelling her contract - "he would take care of everything". When nothing happened, she called him at work and they had "unpleasant words. She was reported to have threatened to make public their affair."

Hoover, keeper of America's secrets, was obsessed with the private life of celebrities, particularly those with leftist leanings. The files show the FBI tracked Monroe from the Cold War mid-1950s to her death in 1962, but particularly after she met and married the playwright Arthur Miller, who was being watched as a possible communist.