If his cell were at Guantanamo Bay, the prisoner would be just one of hundreds of suspected terrorists detained offshore, where the U.S. says the Constitution does not apply.

    But Ali Saleh Kahlah al-Marri is a U.S. resident being held in a South Carolina military brig; he is the only enemy combatant held on U.S. soil. That makes his case very different.

    Al-Marri's capture six years ago might be the Bush administration's biggest domestic counterterrorism success story. Authorities say he was an al-Qaida sleeper agent living in middle America, researching poisonous gasses and plotting a cyberattack.

    To justify holding him, the government claimed a broad interpretation of the president's wartime powers, one that goes beyond warrantless wiretapping or monitoring banking transactions. Government lawyers told federal judges that the president can send the military into any U.S. neighborhood, capture a citizen and hold him in prison without charge, indefinitely.

    There is little middle ground between the two sides in al-Marri's case, which is before a federal appeals court in Virginia. The government says the president needs this power to keep the nation safe. Al-Marri's lawyers say that as long as the president can detain anyone he wants, nobody is safe.

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    A Qatari national, al-Marri came to the U.S. with his wife and five children on Sept. 10, 2001 - one day before the terrorist attacks in New York and Washington. He arrived on a student visa seeking a master's degree in computer science from Bradley University, a small private school in Peoria, Ill.

    The government says he had other plans.

    According to court documents citing multiple intelligence sources, al-Marri spent months in al-Qaida training camps during the late 1990s and was schooled in the science of poisons. The summer before al-Marri left for the United States, he allegedly met with Osama bin Laden and Sept. 11 mastermind Khalid Sheikh Mohammed. The two al-Qaida leaders decided al-Marri would make a perfect sleeper agent and rushed him into the U.S. before Sept. 11, the government says.

    A computer specialist, al-Marri was ordered to wreak havoc on the U.S. banking system and serve as a liaison for other al-Qaida operatives entering this country, according to a court document filed by Jeffrey Rapp, a senior member of the Defense Intelligence Agency.

    According to Rapp, al-Marri received up to $13,000 for his trip, plus money to buy a laptop, courtesy of Mustafa Ahmad al-Hawsawi, who is suspected of helping finance the Sept. 11 attacks.

    A week after the attacks, Congress unanimously passed the Authorization for Use of Military Force. It gave President Bush the power to "use all necessary and appropriate force" against anyone involved in planning, aiding or carrying out the attacks.

    The FBI interviewed al-Marri that October and arrested him in December as part of the Sept. 11 investigation. He rarely had been attending classes and was failing in school, the government said.

    When investigators looked through his computer files, they found information on industrial chemical suppliers, sermons by bin Laden, how-to guides for making hydrogen cyanide and information about chemicals labeled "immediately dangerous to life or health," according to Rapp's court filing. Phone calls and e-mails linked al-Marri to senior al-Qaida leaders.

    In early 2003, he was indicted on charges of credit card fraud and lying to the FBI. Like anyone else in the country, he had constitutional rights. He could question government witnesses, refuse to testify and retain a lawyer.

    On June 23, 2003, Bush declared al-Marri an enemy combatant, which stripped him of those rights. Bush wrote that al-Marri possessed intelligence vital to protect national security. In his jail cell in Peoria, however, he could refuse to speak with investigators.

    A military brig allowed more options. Free from the constraints of civilian law, the military could interrogate al-Marri without a lawyer, detain him without charge and hold him indefinitely. Courts have agreed the president has wide latitude to imprison people captured overseas or caught fighting against the U.S. That is what the prison at Guantanamo Bay in Cuba is for.

    But al-Marri was not in Guantanamo Bay.

    "The president is not a king and cannot lock people up forever in the United States based on his say-so," said Jonathan Hafetz, a lawyer who represents al-Marri and other detainees. "Today it's Mr. al-Marri. Tomorrow it could be you, a member of your family, someone you know. Once you allow the president to lock people up for years or even life without trial, there's no going back."

    Glenn Sulmasy, a national security fellow at Harvard, said the issue comes down to whether the nation is at war. Soldiers would not need warrants to launch a strike against invading troops. So would they need a warrant to raid an al-Qaida safe house in a U.S. suburb?

    Sulmasy says no. That's how Congress wrote the bill and "if they feel concerned about civil liberties, they can tighten up the language," he said.

    That would require the politically risky move of pushing legislation to make it harder for the president to detain suspected terrorists inside the U.S.

    Al-Marri is not the first prisoner who did not fit neatly into the definition of enemy combatant.

    Two U.S. citizens, Yaser Esam Hamdi and Jose Padilla, were held at the same brig as al-Marri. But there are differences. Hamdi was captured on an Afghanistan battlefield. Padilla, too, fought alongside the Taliban before his capture in the United States.

    By comparison, al-Marri had not been on the battlefield. He was lawfully living in the United States. That raises new questions.

    Did Congress really intend to give the president the authority to lock up suspected terrorists overseas but not those living here?

    If another Sept. 11-like plot was discovered, could the military imprison the would-be hijackers before they stepped onto the planes?

    Is a foreign battlefield really necessary in a conflict that turned downtown Manhattan into ground zero?

    Also, if enemy combatants can be detained in the U.S., how long can they be held without charge? Without lawyers? Without access to the outside world? Forever?

    These questions play to two of the biggest fears that have dominated public policy debate since Sept. 11: the fear of another terrorist attack and the fear the government will use that threat to crack down on civil liberties.

    "If he is taken to a civilian court in the United States and it's been proved he is guilty and it's been proved there's evidence to show that he's guilty, you know, he deserves what he gets," his brother, Mohammed al-Marri, said in a telephone interview Friday from his home in Saudi Arabia. "But he's just been taken there with no court, no nothing. That's shame on the United States."

    Courts have gone back and forth on al-Marri's case as it worked its way through the system. The last decision, a 2-1 ruling by a 4th U.S. Circuit Court of Appeals panel, found that the president had crossed the line and al-Marri must be returned to the civilian court system. Anything else would "alter the constitutional foundations of our Republic," the judges said.

    The full appeals court is reviewing that decision and a ruling is expected soon. During arguments last year, government lawyers said the courts should give great deference to the president when the nation is at war.

    "What you assert is the power of the military to seize a person in the United States, including an American citizen, on suspicion of being an enemy combatant?" Judge William B. Traxler asked.

    "Yes, your honor," Justice Department lawyer Gregory Garre replied.

    The court seemed torn.

    One judge questioned why there was such anxiety over the policy. After all, there have been no mass roundups of citizens and no indications the White House is coming for innocent Americans next.

    Another judge said the question is not whether the president was generous in his use of power; it is whether the power is constitutional.

    Whatever the decision, the case seems destined for the Supreme Court. In the meantime, the first military trials are set to begin soon against detainees at Guantanamo Bay. Al-Marri may get one, too. Or he may get put back into the civilian court system. For now, he waits.