An American citizen fighting for the so-called Islamic State is currently in the U.S. military’s custody. If Donald Trump decides to keep him there, it could spark a far-reaching legal challenge that could have a catastrophic effect on the entire war against ISIS, leading national security lawyers say.

The unnamed American, whom the Pentagon says surrendered to U.S.-allied forces battling ISIS in Syria around Sept. 12, is currently held by the military as an enemy combatant. Multiple officials told The Daily Beast that the Trump administration has yet to decide whether that will be his long-term fate.

Neither the Defense Department nor the Justice Department would comment Monday on active deliberations concerning whether the American will face criminal charges in the United States or remain a military detainee. “The disposition for any detainee is ultimately determined by what best supports the national security of the United States and of our allies and partners, consistent with domestic and international law,” said the Pentagon’s detentions spokesman, Maj. Ben Sakrisson.

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