A newly declassified document shows that J. Edgar Hoover, the longtime director of the Federal Bureau of Investigation, had a plan to suspend habeas corpus and imprison some 12,000 Americans he suspected of disloyalty.

Hoover sent his plan to the White House on July 7, 1950, 12 days after the Korean War began. It envisioned putting suspect Americans in military prisons.

Hoover wanted President Harry S. Truman to proclaim the mass arrests necessary to “protect the country against treason, espionage and sabotage.” The F.B.I would “apprehend all individuals potentially dangerous” to national security, Hoover’s proposal said. The arrests would be carried out under “a master warrant attached to a list of names” provided by the bureau.