In 1942, while in a the army, Pete Seeger, the renowned folk musician who died only last year, wrote a letter to protest the Japanese interment camps during World War II. Ever since that letter was reported to the FBI by the California American Legion, Seeger was closely watched and followed. In a time when communists were feared and targeted by the US government, Seeger was a likely target, given his liberal political opinions and evocative lyrics.
Read about the Seeger investigation here. The documents were recently released at the request of Mother Jones.