1. The Mob:
The Mob did it because they were mad at the Kennedys. Giancana had expected JFK's allegiance because the mobster had once saved Joe Kennedy Sr. from a hit during Prohibition days. Instead, Robert Kennedy went gunning for organized crime. Giancana had Marilyn killed to embarrass RFK, but RFK foiled the plan by removing compromising evidence.Also theories that Johnny Roselli appeared at her house to distract her while the hitmen snuck in the back door is another theory.

2. Robert Kennedy:
RFKaided by agents of the Secret Service and CIA injected Marilyn with a fatal dose of Nembutal.JFK and Joe Kennedy are said to have arranged the hit, as her calls to the White House and Justice Department were causing trouble. She not only jeopardized the Kennedy dynasty but also national security, as Bobby had blabbed information about the Castro hit to Marilyn. They asked Giancana to take care of her and he agreed, wanting to be as close to the Kennedys as possible.

Bobby arrived at the place, she was already in overdose mode so he took her to the hospital and she died in the ambulance. He couldn’t be the one to take her dead to the hospital so he went back to the house and called Robert Greenson who officially made the call of her death. Peter Lawford was pulled over by police office, Lynn Franklin on the night Marilyn’s death, and he had Bobby in the backseat – who was in LA according to the official record.

On the tapes from the night of her death, you hear Peter, Marilyn and RFK talking and then fighting, with Peter acting as referee. There is a struggle that indicates that Marilyn was dead when RFK left. The phone then rings, someone picks it up and says nothing. Marilyn was found with the phone in her hand, which means that someone attempted to create a phone record, to prove that she was still alive but not in the state to talk, in order to clear RFK.

Marilyn was taking Enema to control her weight. The book, “Double Cross” suggests that the killers lurked in the bushes, waited until RFK left after sedating her and injected the lethal suppository, in order to frame him.

3. Dr. Ralph Greenson:
The theory that Dr Ralph Greenson, Monroe's shrink, was put forth in "Peter Lawford: The Man Who Kept the Secrets," written by James Hall, who claims to have been the ambulance driver called to the scene following the discovery of Monroe's body. As Hall puts it, "Just as Marilyn started coming around, the doctor arrived... I believe it was Dr. Greeson. He... pushed her breast to one side and gave her an injection."

4. J.Edgar Hoover
Marylin Monroe's entire house was bugged -audio and video in order to gain information on the Kennedys and Giancana, as they were frequent visitors to her home. Incidentally, the guy who bugged her house died in prison, to keep the rumour that her murder was actually videotaped, out of the realm of absolute proof. J. Edgar Hoover and his Mafia cohorts could be directly responsible for the murder of Marilyn Monroe as she associated with the likes of Johnny Roselli, who was angry at the Kennedys for giving the Mafia a hard time and who bragged about "friends in high places" who were going to knock Kennedy off. These assassination rumours invariably disturbed Marylin Monroe, but her frantic attempt to warn the Kennedys was obviously thwarted. If there is one consistent element to the Kennedy assassination it is this: anybody who knew too much was murdered to keep the truth as obscure as possible. Marilyn was just one of the people that had to be silenced.

5. Marilyn herself:
The 1962 Los Angeles County Coroner's report, which listed her cause of death as a suicide.

HOWEVER;

If Marilyn Monroe was murdered by the Kennedy’s because she knew too much, why didn’t someone murder Judy Campbell? She no doubt had incriminating evidence against the Kennedys, no doubt more information that Marilyn did. The same goes for Giancana and co. If they were involved in the Kennedy assassination, why didn’t they finish the job and rub out Campbell? She has information on Giancana’s involvement with the Kennedy’s, she was running money and arranging meetings between the two – she has information on the mob connections to the white house. Why did she die of cancer instead of a mob hit?

Marilyn was said to have died from an overdose of sleeping pills, however, there was a lack of pill residue in her stomach and no water to down the amount of pills she was supposed to have taken. The autopsy also discovered that a large section of her intestine, the sigmoid colon, was discolored. This part of the intestine is not far from where an outside source would be induced to the body by way of an enema. A lethal enema wouldn't leave any residue in the stomach. Many years later when asked what really happened to Monroe, Peter Lawford said, "Let's just say, poor Marilyn took her last big enema."