The detention of an unnamed U.S. citizen held by the military as an enemy combatant is increasingly a legal and human-rights emergency, national security lawyers and civil libertarian activists say—and a bellwether case for the unsettled question of what terrorism captures in Donald Trump’s administration will look like.

The executive director of the American Civil Liberties Union, Anthony Romero, wrote on Friday to Attorney General Jeff Sessions and Defense Secretary Jim Mattis to urge the U.S. citizen’s transfer to civilian custody, calling his military detention “unlawful as a matter of domestic law.”

“The course you take with respect to the U.S. citizen in Defense Department custody is a critical test for this administration’s adherence to the rule of law,” Romero wrote in a letter obtained by The Daily Beast. “I accordingly urge you to transfer him to the United States and the federal criminal justice system without delay.”

On September 14, The Daily Beast broke the story of an American captured fighting for the so-called Islamic State by Syrian allies of the U.S. Hours after the story was published, the U.S. military acknowledged detaining him since his September 12 “surrender” as an enemy combatant.

Since then, Justice Department and Pentagon officials have released almost no information on him—not even his name, which is key for family or rights groups seeking to challenge his military detention. Government officials are deliberating what to ultimately do with him, without evident resolution, creating significant uncertainty as to whether he will be tried in criminal court—which was the Supreme Court-compelled policy of George W. Bush after 2004 for U.S. citizens taken by the military, and Barack Obama’s preferred policy.

Nearly nine months into the Trump administration, a long-rumored executive order setting detentions policy has yet to manifest, something a knowledgeable Trump administration official said was the result of internal dysfunction.

As a result, figuring out what to do with the captured American citizen is “the Gordian knot of craziness,” the official told The Daily Beast. There is a prospect that Trump could drift into opting for indefinite military detention, which the official acknowledged “would be the de facto template, but not because of some studied plan.”

On Thursday, both the Pentagon and the International Committee for the Red Cross acknowledged publicly that they are negotiating Red Cross access to the citizen, whose location is reportedly Iraq but not officially acknowledged. That means the Red Cross has not seen him more than two weeks after what the military characterized as his surrender.

“Until he meets with the ICRC, he is effectively disappeared,” said Wells Dixon of the Center on Constitutional Rights.

Trump’s lack of a decision is raising what Dixon called “increasing concern among the human rights community and the legal community in general about what’s happening with this individual.” Many are also concerned that the case has received little attention from politicians and journalists, despite its wide-ranging implications.

Several attorneys interviewed for this story believe that the legal jeopardy the Trump administration will create for itself by keeping him in military custody is so massive that it will eventually have to pursue his case in federal court.

But they do not know whether the mercurial Trump will see it that way. Among the only statements the Pentagon has made is an assertion that it has the right to hold citizens as enemy combatants. “There is nothing that prohibits detaining a U.S. citizen as an enemy combatant at Guantanamo,” its detentions spokesman, Maj. Ben Sakrisson told The Daily Beast last week —a position legal scholars dispute.

“ “Until he meets with the ICRC, he is effectively disappeared.” ” — Wells Dixon of the Center on Constitutional Rights

Though the U.S. will not formally acknowledge it, the unnamed citizen is likely facing some form of interrogation, either for intelligence purposes or, should he be Mirandized, for criminal prosecution. But each passing day without a dispensation for his case, or an announcement of what that dispensation will be, raises the prospect that Trump, whether through deliberation or drift, will make indefinite military detention of U.S. citizens captured overseas official U.S. policy.

“Time is running out for the Pentagon to transfer this detainee to the Justice Department for a criminal investigation, if it hasn’t run out already. While the military may question this US citizen on intelligence before handing him over to the criminal justice system, that process should wrap in days, not weeks,” said Letta Tayler, a senior counterterrorism researcher at Human Rights Watch.

“There is an elusive but not illusory point when this crosses over from short-term detention to long-term detention, and when the legal issues become incredibly serious,” added Steve Vladeck, a national-security specialist at the University of Texas law school. “I’m not sure we’re there yet, but we’re getting close in a hurry, and the more the government tries to draw this out and forestall litigation, the more it’s raising the stakes of the potential legal disputes to come.”

Those stakes are high. As The Daily Beast reported last week, if he challenges his military detention, its legality is at stake. And since the legality of his detention stems from the 2001 and 2002 Authorizations to Use Military Force, both of which were passed before ISIS existed, a federal judge could rule that the entire war against ISIS is illegal. Until now, no one possessed the legal standing to challenge the foundations of the anti-ISIS war. All lawyers interviewed for this story agree that a U.S. citizen captured fighting for ISIS does.

According to the Trump administration official, one reason the detentions executive olrder is delayed is because some in the national-security apparatus argued against specifying its application to ISIS without a firmer legal grounding in domestic law than the AUMF provides. Yet in August, the administration ruled out seeking “additional authorizations” from Congress.

“The Trump administration would be foolish to create a situation, for the sake of this single detainee, in which a federal court for the first time could weigh in on the relevance of the 2001 and 2002 AUMFs as to the Islamic State,” said Bobby Chesney, another University of Texas national-security law professor and the co-founder of the influential legal blog Lawfare.

But without the citizen’s name being public, it will be extraordinarily difficult for anyone to lodge a habeas corpus challenge to his military detention. That raises the prospect that the Trump administration possesses an incentive to keep his name a secret—and the citizen functionally beyond the reach of the law—should it choose to keep him in military custody.

Few believe that the U.S. could keep from releasing his name forever, whether by official disclosures, leaks to the press or a statement from a notified family member. In general, said Marc Kilstein, a spokesman for the ICRC, the group “may facilitate contact between detainees and their relatives, if required.” That increases the likelihood of the citizen’s name emerging after the ICRC’s upcoming visit.

“Delayed disclosure of the person’s identity has thus far ensured that litigation has not begun. Disclosure cannot be withheld permanently, however, in the case of a citizen at least; otherwise habeas jurisdiction always could be defeated through simple silence. All of which suggests that prosecution is a much more likely alternative, sooner or later,” Chesney said.

But if Trump decides against that alternative, Dixon said, the Center for Constitutional Rights is girding for a courtroom fight.

“ What that means is the legality of this individual’s military detention will be challenged in federal court. The longer this person is held in U.S. military custody the stronger the legal challenge will be,” Dixon said.