Pentagon officials raised the possibility on Thursday of indefinite detention of prisoners from the Afghan war, saying that captives might not be released from US custody even if they were acquitted in a military tribunal.
In addition, they said, prisoners may be held for the duration of the war, and there is no end in sight.
US Secretary of Defense Donald Rumsfeld has said previously that the 500 prisoners in custody -- 300 in Guantanamo Bay, Cuba, and nearly 200 in Afghanistan -- could be held indefinitely, but this possibility was given a new twist on Thursday as Rumsfeld and top aides announced the rules by which military tribunals would be conducted.
"If we had a trial right this minute, it is conceivable that somebody could be tried and acquitted of that charge but may not necessarily automatically be re-leased," said William Haynes, the Pentagon's top lawyer, calling the prisoners "dangerous people."
The tribunals, which would be mostly open to reporters, would have three to seven members and require a unanimous vote for imposing the death penalty, rather than the two-thirds vote that US President George W. Bush had called for originally in establishing the tribunals on Nov. 13. They also loosen the rules of evidence, allowing hearsay and do not provide an independent appeals process.
Rumsfeld said: "If one steps back from examining the procedures provision by provision and instead drops a plumb line down through the center of them all, we believe that most people will find that, taken together, they are fair and balanced and that justice will be served in their application."
But some human rights groups were in an uproar over the rules.
Jamie Fellner, a spokeswoman for Human Rights Watch, said: "Not to have an independent court of appeals and then to have the president have the final say potentially undercuts whatever fairness they have sought to provide at the trial level."
The rules drew little criticism from Congress.
Senator Patrick Leahy, a Democrat and chairman of the Judiciary Committee, said that he had commended Rumsfeld on Thursday for "taking steps that have substantially improved this proposal."
Senator Arlen Specter, a Republican, said the rules were "on the right track" but he still wanted them to be approved by Congress. He has introduced legislation offering slightly different rules, and said he wants hearings on the matter.
Haynes said that it was too early to tell who might face the tribunals because it was difficult to gather evidence from the battlefield and from the prisoners.
"They're singularly uncooperative," Haynes said.
Other senior administration officials said that perhaps a dozen or two dozen of the 500 prisoners in US custody would be likely candidates.
"The purpose is to try people who have committed the most significant violations of the laws of war, and there may be other ways to address other people, including transferring them to other countries," one official said.
He said that the administration expected the trials would not begin for several months because of the laborious process of building a case. "Each of these trials will be very difficult to do," the official said.
During the briefing, Haynes and Douglas Feith, undersecretary for policy, said it was possible that someone could be acquitted in a tribunal but not set free. Feith said that as in civilian courts, a prisoner might be acquitted on one charge "but there still may be other reasons to hold the person."