Good old J. Edgar Hoover's witch hunt. The late screen siren was one of several Hollywood stars the FBI kept tabs on, speculating on their affiliation with the Communist party. Other notable figures included Louis Armstrong, Frank Sinatra, John Lennon, the Beach Boys and Lucille Ball. In newly released documents, Monroe fell under the gaze of the FBI when applying for a visa to travel to Russia and was subsequently tracked meeting with a Communist group in Mexico. Sinatra perked suspicions after attending sex parties with President Kennedy. Now that's your grandparents tax dollars hard at work.

Source: IMDB