What controversial step was taken by President Wilson's Attorney General Mitchell Palmer in 1920?

Palmer launched a campaign against radicals known as the Palmer Raids. Suspects were arrested with no evidence of their having committed any crimes, but on their affiliations with radical groups. This lead to the May Day warning of 1920 when Palmer warned the nation to expect the worst - assassinations, bombings and general strikes. Law enforcement was mobilized in cities such as New York and Boston, but May Day came and went with no violence.