According to an answer on this question on the WorldBuilding StackExchange
There were plenty of communists in the United States in the 1950s and 1960s. It's popular today to laugh at the paranoia of Joe McCarthy et al. But in fact when the Soviet Union fell and westerners were able to access KGB files, it was found that most of the people McCarthy accused of being communists were, in fact, paid spies in contact with the Soviet government. One book I read claimed that literally EVERYONE that McCarthy accused turned out to be either a member of the Communist Party, a paid Soviet agent, or in contact with the Soviet government. I haven't researched it to find out if that's true. But it's clear that the Soviets did have a systematic program to plant agents in the U.S. government, and that even without that, there were plenty of Americans who were communist "true believers". It's not that hard to imagine that if these efforts, by Soviet and by American communists, had just been a little more successful, that communists might have taken over the government -- won a majority in Congress, elected a president, etc -- the U.S. could have turned communist, the two sides declared unity, and the Cold War ended with a communist victory.
I could find no references to the suggestion that many people accused by McCarthy were actually paid spies. However, I could not find any conclusive evidence either way.
I wish to know if the claims of McCarthy's accusee's being actual spies truthful.