The Federal Bureau of Investigation
The FBI had a strong anti-Communist director, J Edgar Hoover. He used his FBI loyalty boards to investigate government employees to see if they were current/former members of the Communist Party. From 1947-1950, around 3 million were investigated but no-one was charged with spying or espionage.
The House of Un-American Activities Committee
This committee had the right to investigate anyone who was suspected of doing anything "un-American", yet it was fairly unheard of but in 1947 it became big news.The FBI had evidence that a number of prominent Hollywood writers, producers and directors were involved with the Communist Party and they were questioned. However being a member of the Communist Party in a free democratic country such as the USA wasn't illegal, so when the Hollywood Ten as they were known, appeared before the committee, they refused to answer any questions. Instead they pleaded the First Amendment of the US constitution (all Americans have the freedom to believe what they want) and said that the…