The court is undertaking a fresh review of the Bush administration's aggressive use of preventive detention for suspected terrorists. The administration asserts that the president has the authority to order the military to detain anyone suspected of being an al-Qaida member.

The administration's policy is being challenged by Ali al-Marri, a Qatar native who was seized in the United States and is the only enemy combatant currently being held on U.S. soil. The government says al-Marri is an al-Qaida sleeper agent.

Al-Marri, represented by the American Civil Liberties Union, says he cannot be imprisoned without charge or trial. He was arrested in Peoria, Ill.

Al-Marri has been held in virtual isolation in a Navy brig near Charleston, S.C., for nearly 5 1/2 years.

The 4th U.S. Circuit Court of Appeals in Richmond, Va., said in a split decision that the president has such power, but that al-Marri must be given the chance to persuade a federal judge that he is not an enemy combatant.

The administration argued that al-Marri's case should first go to federal district court in South Carolina, instead of to the Supreme Court.

Al-Marri said that the case was of such constitutional importance that it should be heard by the high court now — and the justices apparently agreed.