Final answer:
To uncover communists in prominent positions, several U.S. government entities were involved. President Truman's Executive Order 9835 led to FBI-led loyalty reviews under J. Edgar Hoover, and Congressional committees like HUAC conducted their own investigations.
Step-by-step explanation:
Initiation of the Probe to Uncover Communists
The movement to ferret out communists in prominent positions was led by a number of entities within the U.S. government. President Truman was responsible for issuing Executive Order 9835 in 1947, which laid the foundation for the loyalty reviews of federal employees, a process significantly spearheaded by the Federal Bureau of Investigation (FBI), under the direction of J. Edgar Hoover. The FBI played a central role in investigating these employees and determining potential security threats. Additionally, the House Un-American Activities Committee (HUAC) and the Senate Permanent Subcommittee on Investigations were instrumental in holding investigations and hearings regarding communist influence in the United States.
In the wake of these actions, several government and non-government entities, including state and municipal loyalty boards, were involved in identifying and dismissing individuals believed to be disloyal or a security risk. These extensive investigations often resulted in the ruining of careers, not just based on evidence of communism, but also due to associations with various civil rights and anti-war movements.