The same source.
'We got back at 11.30 at night. She didn't stay at the White House, she stayed somewhere else. There was no hanky-panky between her and JFK that night. I know, because I asked him the next day and he would have happily said so.
'But, anyway, Jackie knew about that trip. In fact, we were dancing at a White House ball and she said to me: "Don't think I'm naive about what you and Jack are doing with all those pretty girls, like Marilyn, sailing on the Potomac under the moonlight."
'She was accustomed to Kennedy's indiscretions, but this one bothered her. She knew from what she'd heard and read that Marilyn was a troubled woman. It wasn't so much that she was angry about it, more that she was just disgusted.'
Sadly, Jackie was the only person close to the President who had realised - or voiced concern about - Marilyn's vulnerability. Even the briefest glance at her psychological history should have set off alarm bells among Kennedy's aides that this was one woman not to be seduced and discarded like so many other of his conquests.
The scars of Marilyn's past were visible to anyone who chose to look, and her impulsive and drug-fuelled behaviour repeated itself throughout all her relationships
By 1962, the year she died, she was heavily dependent on prescription drugs and alcohol to control her emotional instability. She feared she had inherited her mother's paranoid schizophrenia and often heard 'voices' or claimed she was being followed.
Since childhood, she had been unable to cope with any form of rejection or criticism and, on a number of occasions, took pills in apparent suicide attempts. Insecure, needy and with three failed marriages, she was desperate to find a man to compensate for the lack of a father figure in her disrupted childhood.
Her first marriage, at the age of 16, had been to a neighbour's son, Jim Dougherty, whom she called, bizarrely, Daddy. In the early days of their marriage she told Jim she thought she was being followed, and she began to use threats of suicide to gain attention: 'If anything bad happens to our marriage,' she would warn him, 'I'll jump off a bridge, Daddy.'
The marriage failed when Marilyn's photogenic potential was spotted and she was encouraged to try her luck in the ruthless Hollywood studio system. She was noticed in 1949 by Johnny Hyde, a powerful agent with the William Morris agency. He became obsessed with her; she, in turn, saw him as a powerful father figure who, more importantly, had a way of fixing the voices she heard in her head.
He believed that barbiturates would help, and at his behest studio doctors began prescribing drugs to her on a regular basis. They did help - at least, in the short term. Her anxieties decreased and the voices became softer and bothered her less. It was the start of a lifelong relationship with medication.
But in December 1950 Johnny Hyde died of a heart attack. A couple of days after his funeral, Marilyn's acting coach Natasha Lytess found her collapsed in her bedroom. Horrified, Natasha rushed to her side and forced open her mouth. It was full of dissolving pills.
Her third marriage, in June 1956, was to yet another father figure - the intellectual playwright Arthur Miller, who, at 40, was older than her by a decade. But a miscarriage, followed by a heartless letter from her mentally unstable mother Gladys - telling her that along with motherhood came certain responsibilities, 'and you, dear child, are not a responsible person' - was the catalyst for Marilyn's increasing dependence on drink and drugs.
One night, Miller had a phone call from her while he was in a restaurant; she said he had to come home and save her. She had taken an overdose.
Edward Lovitz, a screenwriter who had known Miller for many years, said: 'Arthur told me that he thought she needed psychiatric help, that she would start to scream at him for no apparent reason. He wasn't sure if it was the drugs she was taking, the alcohol or just her mind breaking down on her.'
The end of her marriage to Miller resulted in the next big crisis in Marilyn's life. Alarmed by her suicidal obsession, her psychiatrist Dr Marianne Kris suggested that she check into a private ward at New York Hospital for some rest under medical supervision. Reluctantly, Marilyn agreed.
On Sunday, February 5, 1961, Dr Kris drove her to the hospital, where Marilyn checked in using the pseudonym of Faye Miller. However, when the time came to go to her room, she found herself being taken to another clinic on the premises.
Her journey involved passing through numerous steel doors, most of which required a key from both sides. Suddenly, it all became clear: those doors were meant to keep people in, not keep people out - she was in the Payne Whitney Clinic, the psychiatric division of the hospital.
After a hysterical tantrum, she was forcibly thrown into a sparse padded room with barred windows. She banged her fists against the hard metal door until two nurses came. If she persisted, they warned her, she would be put into a straitjacket. They turned off the light, leaving their stunned patient in the dark - and without her medication.
She hurled a chair against the glass on the bathroom door, then extracted a small, sharp sliver from the cracked window. As a team of doctors and nurses burst into her room, she held the jagged glass to her wrist. 'If you don't let me out of here, I'll ***********,' she threatened.
Monroe was taken to a floor for seriously disturbed patients, where a young doctor came in to see her. 'Why are you so unhappy?' he asked her. Marilyn answered: 'I've been paying the best doctors a fortune to find out the answer to that question, and you're asking me?'
On Thursday morning, she was allowed to make one phone call. She knew she would have to contact someone who would move heaven and earth to get her out of that place. Who was the most obstinate man she knew? Her former husband, baseball star Joe DiMaggio.
Their marriage hadn't ended well, but based on the kind of man he was, she knew she would be able to count on him.
His friend Stacy Edwards recalled: 'Joe told me he's sitting in his motel room in Florida having a cold beer and watching TV when the phone rings. It's Marilyn, sobbing that she's in a nuthouse in New York and she needs him to get her out of there.
'He thought it was a joke. He said she was making no sense at all and he thought it was a prank, or she was high on pills and delusional. But after he calmed her down, she told him she needed him. That was all he needed to hear. He jumped on the next plane.'