For many years, the Federal Bureau of Investigation and other agencies spied on Albert Einstein, acting on suspicions as disturbing as a tip that he had been a Russian spy in Berlin; as vague as an unease with his support of civil rights and pacifist and socialist causes; and as goofy as claims that he was working on a death ray or that he was heading a Communist conspiracy to take over Hollywood.

The investigation turned up nothing. Nevertheless, the agency dogged Einstein’s footsteps until his death in 1955, even cooperating with an investigation by the Immigration and Naturalization Service to see whether he should be deported.
More disturbing stuff about this here.