Why were the “hollywood ten” sent to prison?

Originally, 40 people were suspected of holding Communist loyalties  or being involved in harmful activities (the main reason the Hollywood Ten were arrested). The "Hollywood Ten" denied the HAUC hearings, claiming it was a violation to their Civil Rights. Some even comparing the hearings to the actions of Nazi, Germany. One of the Hollywood Ten, Lawson, commented "I am not on trial here, this committee is on trial."

*Hope this helped, if not let me know! Here is my main source.

         https://www.history.com/topics/cold-war/hollywood-ten

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