It is a question that troubles civil liberties advocates - even those who support the Bush Administration's detention policies in general.

Does the government have the right to indefinitely hold an American citizen or a foreign national here legally?

Ali Saleh Kahlah al-Marri was close to going on trial for fraud when prosecutors marched into an Illinois courtroom with a demand. Dismiss the charges, they said, because President Bush had just designated the defendant an enemy combatant.

Marri's attorneys protested, but U.S. Attorney Jan Paul Miller declared that the military had already taken custody of the Qatari national, now deemed an al-Qaeda sleeper agent. "There is no longer a judicial proceeding before this court," he said.

With that, Marri was whisked to a Navy brig in Charleston, S.C., where he has spent more than five years. His case raises a question with vast implications for presidential power and civil liberties: Can the military indefinitely detain, without charge, a U.S. citizen or legal resident seized on U.S. soil?

The Supreme Court is now being asked to consider the legality of Marri's detention, which is one of the broadest and most controversial assertions of executive authority since the Sept. 11, 2001, attacks. Marri's attorneys want the court to overturn an appellate ruling that backed the administration. The final brief is due Monday, and the justices are expected to decide soon whether to take the case.