I’m not clear whether this is based on known or unseen files, but FBI material on MM tends to be highly speculative.
Here’s an extract from an article by Kessler, writing for The Daily Beast:
“Many of the confidential files were destroyed after Hoover’s death. One such item that never came out previously was a teletype sent to headquarters from William Simon, who headed the Los Angeles field office, just after the August 5, 1962, death of Marilyn Monroe at her Brentwood, California home. According to Cartha ‘Deke’ DeLoach, who saw the teletype, it said that then Attorney General Robert Kennedy had borrowed Simon’s personal car to see Monroe just before her death.
Confirming this, Simon’s son Greg says, “My father said Robert Kennedy would borrow his white Lincoln convertible. That’s why we didn’t have it on many weekends.” Simon’s daughter Stephanie Branon also confirmed that her father lent his car to Kennedy and remembered that the attorney general once left his Ray-Ban sunglasses in the glove compartment.
As attorney general, Kennedy was entitled to be driven by an FBI security detail. The fact that he chose to use Simon’s personal car is consistent with William Simon’s report to headquarters that he lent his car to Kennedy for the purpose of clandestine meetings with Monroe. Whether his last meeting with her, possibly to break up with her, may have contributed to her suicide is legitimate speculation.”