Attorney General A. Mitchell Palmer, with a young J. Edgar Hoover directing the Justice Department's new Radical Division, answered the 1919 bombings with the largest mass roundup in American history: coordinated raids in November 1919 and January 1920 swept up thousands of immigrant radicals in dozens of cities — estimates run from three to ten thousand — most seized without warrants, held incommunicado, and beaten in overcrowded detention. Since mere membership in a proscribed organization was a deportable offense for noncitizens, the government needed no criminal trials at all.

On December 21, 1919, the army transport Buford — the press called it the "Soviet Ark" — sailed from New York with 249 deportees, Emma Goldman and Alexander Berkman the most famous among them. The excesses provoked the founding of the ACLU and a devastating report by twelve prominent lawyers documenting the illegality; Assistant Labor Secretary Louis Post canceled thousands of deportation orders and survived impeachment for it. Palmer's predicted May Day 1920 uprising never came, and the panic collapsed — but the files Hoover built in 1919 became the foundation of fifty years of FBI political surveillance.