The FBI had their suspicions, and recently re-released documents from 1956 to 1962 show just how closely the Bureau was monitoring the star and her associates.
Monroe was said to have "drifted into the communist orbit" by an anonymous phone call to the New York Daily News, which also accused her husband, Arthur Miller was a member of the party.
The tip, received by the FBI on July 11, 1956, said "money from Marilyn Monroe Productions was finding its way into the [Communist Party]" and that the company was "filled with communists."
Monroe and Miller were married on June 29, 1956, and he became a target of the House Un-American Activities Committee shortly thereafter (he was convicted of contempt in May 1957 and cleared in 1958). The caller claimed their marriage was a "cover up" for Miller's communist activities.
The documents also reveal Monroe's friends' growing concerns over her friendship with Frederick Vanderbilt Field, who had been disinherited from his family for leftist views and was living in Mexico in self-imposed exile.
Monroe took a trip in February 1962, six months before she died, ostensibly to shop for furniture, meeting with Field. Their "mutual infatuation" caused "considerable dismay among Miss Monroe's entourage and also among the American Communist Group in Mexico," the file said.