Hoover’s plan was declassified Friday as part of a collection of cold-war documents concerning intelligence issues from 1950 to 1955. The collection makes up a new volume of “The Foreign Relations of the United States,” a series that by law has been published continuously by the State Department since the Civil War.

Hoover’s plan called for “the permanent detention” of the roughly 12,000 suspects at military bases as well as in federal prisons. The F.B.I., he said, had found that the arrests it proposed in New York and California would cause the prisons there to overflow.

So the bureau had arranged for “detention in military facilities of the individuals apprehended” in those states, he wrote.

The prisoners eventually would have had a right to a hearing under the Hoover plan. The hearing board would have been a panel made up of one judge and two citizens. But the hearings “will not be bound by the rules of evidence,” his letter noted.

The only modern precedent for Hoover’s plan was the Palmer Raids of 1920, named after the attorney general at the time. The raids, executed in large part by Hoover’s intelligence division, swept up thousands of people suspected of being communists and radicals.

Previously declassified documents show that the F.B.I.’s “security index” of suspect Americans predated the cold war. In March 1946, Hoover sought the authority to detain Americans “who might be dangerous” if the United States went to war. In August 1948, Attorney General Tom Clark gave the F.B.I. the power to make a master list of such people.

Hoover’s July 1950 letter was addressed to Sidney W. Souers, who had served as the first director of central intelligence and was then a special national-security assistant to Truman. The plan also was sent to the executive secretary of the National Security Council, whose members were the president, the secretary of defense, the secretary of state and the military chiefs.

In September 1950, President Truman vetoed as unconstitutional a law authorizing the detention of “dangerous radicals” if the president declared a national emergency, but Congress voted overwhelmingly to override his veto and approve it. Truman did declare such an emergency in December 1950, after China entered the Korean War. But no known evidence suggests he or any other president approved any part of Hoover’s proposal.