New details about the unidentified detainee, held without charge in Iraq by the U.S. for nearly five months, were included in documents the American Civil Liberties Union filed in U.S. District Court in Washington.
If the U.S. government wants to continue to detain him, it must charge him with a crime, the ACLU told the court.
The ACLU said the man, who surrendered on the Syrian battlefield to U.S.-backed forces, claims the allegations against him are "riddled with inaccuracies." The U.S. military is detaining him in Iraq as an enemy combatant.
"Not even the government alleges that he ever took up arms against the United States or anyone else," the ACLU said.
The government has claimed the right to detain him under war powers Congress passed after 9/11 in the fight against the Taliban and al-Qaida, the 2002 congressional authorization for the war in Iraq and presidential power to imprison Americans indefinitely for national security reasons. The ACLU argues those war powers don't apply in the battle against IS.
"The executive cannot circumvent Congress by imprisoning Americans based on statutes authorizing military force for different wars against different adversaries," said ACLU attorney Jonathan Hafetz. "In detaining this U.S. citizen without charge for months on end, the Trump administration is unlawfully reviving one of the most egregious abuses of executive power we saw after 9/11. The administration is also pushing the dangerous claim that President Trump has the independent authority to indefinitely detain Americans at will."
The detainee's fate is unknown, although the government is considering the option of transferring him to a third country, perhaps Saudi Arabia where he has citizenship. He was born in the United States, but grew up in the Middle East.
Last month, U.S. District Judge Tanya Chutkan ordered the U.S. government to give 72 hours advance notice before making any transfer. The ruling doesn't prevent the government from ever transferring the detainee to another country, but says it has to provide advance notice of any transfer so it can be contested in court.
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