Albert Einstein was targeted as a potential spy for the Soviet Union by conservative critics of his liberal political beliefs. Einstein was a supporter of socialist policies and civil rights for people of all races, which was apparently enough to raise suspicion. Other factors, including Einstein's strong opposition to war, his German heritage and his public criticism of Senator Joseph McCarthy, caused U.S. government officials such as FBI director J. Edgar Hoover to regard Einstein with strong suspicion.
No evidence of spying activity was ever uncovered, though the FBI did determine that such a "radical" was unfit to work on official government projects. One of these official projects was the Manhattan Project, the research project that led to the development of the nuclear bomb. The FBI acquired a massive file on Einstein. This file, which ended up totaling almost 2000 pages, is partially available on the FBI's website thanks to the Freedom of Information Act.