Final answer:
J. Edgar Hoover issued a warning about radicalism and communism in the United States, resulting in the Palmer raids.
Step-by-step explanation:
J. Edgar Hoover issued a warning in response to the perceived threat of radicalism and communism in the United States. In 1919, as the head of the Bureau of Investigation (predecessor to the FBI), Hoover organized nationwide raids on radical headquarters and arrested thousands of alleged American radicals. These raids were known as the 'Palmer raids,' named after Attorney General A. Mitchell Palmer.