Source: Library of Congress

The FBI ran an investigation on Albert Einstein. The following are from the FBI website and were released under the Freedom of Information Act: (1427 pages)

An investigation was conducted by the FBI regarding the famous physicist because of his affiliation with the Communist Party. Einstein was a member, sponsor, or affiliated with thirty-four communist fronts between 1937-1954. He also served as honorary chairman for three communist organizations.

The following are PDF files. If you don't have a PDF reader, you can download Acrobat Reader HERE for free. Source: FBI under Freedom of Information Act and contains over 3,800 documents.