Former FBI director J Edgar Hoover had a plan to arrest 12,000 Americans he deemed a possible threat to national security, declassified papers reveal.
The FBI chief sent his proposal to US President Harry Truman just after the start of the Korean War in 1950, The New York Times newspaper reports.
He asked the president to declare the mass arrest necessary to counter "treason, espionage and sabotage".
There is no evidence any part of the plan was ever approved.
Mr Hoover wanted the president to suspend the centuries-old legal right of habeas corpus, which protects individuals against unlawful arrest.
The FBI director planned to detain the suspects - whose list of names he had been compiling for years - in US military and federal prisons.
"The index now contains approximately 12,000 individuals, of which approximately 97% are citizens of the United States," wrote Mr Hoover, in the now declassified document.
The New York Times gave no details about the identities of those targeted.
The US Department of State declassified the plan, along with other Cold War-era documents from 1950-55 this week.