How did the hunt for artists in Hollywood who were associated with communism begin?

You had a lot of people in power for example Senator Joseph McCarthy who would go to congress and tell people that the communists were everywhere and eventually the red scare would begin, McCarthy would famously go to congress and say "I have 200 names of communists in our state department" then when a reporter asked him how many he decreased the number to 150, and then by the end of the day it was 50 communists. But many people where still afraid so they begin trials where communists were rounded up and arrested and thrown into prisons, then the government realized that the media and Hollywood had the most power to influence the population so they began asking people in Hollywood if they new communists etc...