The FBI probably knew beforehand.
One of the most well-known stories of foreknowledge in the Kennedy assassination is that of Rose Cherami (often spelled "Cheramie"), whose real name was Melba Christine Marcades.
On November 20, 1963, Cherami was struck by a car on Highway 190 near Eunice, Lousiana. She told police Lt. Francis Fruge she had been traveling with two men from Florida to Dallas, as part of a drug run, but had been thrown out of the Silver Slipper Lounge after an argument, after which she had been run over. After exhibiting drug withdrawal symptoms, Fruge took her to Jackson East Louisiana State Hospital.
On the journey there, Fruge later told the HSCA, Rose Cherami told the story of her companions and the argument, and then when asked about her business in Dallas, she said she intended to "number one, pick up some money, pick up her baby, and kill Kennedy." She reportedly subsequently told hospital nurses, moments before JFK was killed, that the murder was about to happen. A few days later, she told Fruge that Oswald was a friend of Jack Ruby, for whom she said she worked as a stripper and dope runner.
Dr. Victor Weiss, who treated Cherami, told Jim Garrison's investigators in 1967 that he had heard Rose's predictions about the Kennedy assassination. In his testimony to the HSCA Dr. Weiss was clear that he had heard this before Kennedy's assassination, though the initial 1967 contact report notes that "Dr. Weiss states that he doesn't recall whether this was told to him before or after the assassination."
Rose Cheramie (Cherami) was found unconsciousness by the side of the road at Eunice, Louisiana, on 20th November, 1963. Lieutenant Francis Frugé of the Louisiana State Police took her to the state hospital. On the journey Cheramie said that she had been thrown out of a car by two gangsters who worked for Jack Ruby. She claimed that the men were involved in a plot to kill John F. Kennedy. Cheramie added that Kennedy would be killed in Dallas within a few days. Later she told the same story to doctors and nurses who treated her. As she appeared to be under the influence of drugs her story was ignored.
Following the assassination, Cheramie was interviewed by the police. She claimed that Lee Harvey Oswald had visited Ruby's night club. In fact, she believed the two men were having a homosexual relationship.
Rose Cheramie was found dead on 4th September, 1965. At first it appeared she had been involved in a road accident. Later it was argued that she had been shot in the head before being run over by by a car in order to disguise the original wound. However, the Louisiana State Police Memo reported: "Cheramie died of injuries received from an automobile accident on a strip of highway near Big Sandy, Texas, in the early morning of September 4, 1965. The driver stated Cheramie had been lying in the roadway and although he attempted to avoid hitting her, he ran over the top of her skull, causing fatal injuries. An investigation into the accident and the possibility of a relationship between the victim and the driver produced no evidence of foul play. The case was closed."