The files don't appear to shed any new information on the actress's death 50 years ago. However, the letters and news clippings in the files obtained by the Associated Press, show the bureau was aware of the various theories surrounding her death.
Some of those files, which had been kept secret, do reveal how closely government agents had been monitoring the star before her death in August 1962.
Monroe's file begins in 1955 and mostly focuses on her travels and associations, searching for signs of leftist views and possible ties to communism.
The FBI was suspicious of some of Monroe's communist-leaning friends including her "mutual infatuation" with Frederick Vanderbilt Field, who was living in the US with his wife in self-imposed exile. Field had been disinherited from his wealthy family over his leftist views.
The file continues up until the months before her death, and also includes several news stories and references to Norman Mailer's biography of the actress, which focused on questions about whether Monroe was killed by the government.
Los Angeles authorities found no evidence of foul play in Monroe's death and concluded she died of a probable suicide. Controversy has remained over the star's death since the FBI files have been "heavily censored."
The Associated Press contributed to this report.