The U.S. military confirmed a Daily Beast report Thursday that an American fighting in Syria for the so-called Islamic State has been taken into custody.

“The U.S. citizen is being legally detained by Department of Defense personnel as a known enemy combatant,” Pentagon spokesman Eric Pahon said.

A source familiar with the situation told The Daily Beast earlier that the American was captured by Kurdish forces. Other military spokespeople indicated that the fighter “surrendered” on or around Tuesday.

In either case, the detention of an American fighting for ISIS on an active battlefield would set up a major decision for Donald Trump about the future of wartime captures.

According to The Daily Beast’s source, the U.S. citizen was initially taken into custody by the Syrian Democratic Forces, the mostly Kurdish local proxy that the American military is using to fight ISIS on the ground in Syria. That source said the Kurds then turned the captive over to American forces. It is not clear where the American is currently being held, nor if the International Committee of the Red Cross has had access to him.

Neither the U.S. military command overseeing the war against ISIS nor the Justice Department, which plays a substantial role in deciding what to do with U.S. citizen detainees, are disputing this reporting.

“We are aware of the report that a U.S. citizen believed to be fighting for ISIS surrendered to Syrian Democratic Forces on or about Sept. 12,” said the spokesman, Maj. Earl Brown.

“We defer to the U.S. Department of Justice and the Government of Iraq on this issue. The Coalition’s mission is to defeat ISIS in Syria and Iraq, and we will pursue ISIS fighters regardless of nationality,” he said.

Early Thursday morning, the U.S. military command overseeing the war in Iraq and Syria gave The Daily Beast a new statement that also conspicuously did not deny the capture. Instead, it punted to the State Department and said any detainees would be “transported humanely” to their national authorities’ custody.

“We are aware of the report that a U.S. citizen believed to be fighting for ISIS surrendered to Syrian Democratic Forces on or about Sept. 12. As a precondition for Coalition support, SDF and Iraqi forces have pledged to observe international laws and the laws of armed conflict. Foreign fighters who are captured or surrender to SDF partners in Syria will be safeguarded and transported humanely, and their home nations will be contacted regarding the next steps,” the command statement to the Daily Beast said.

“The Coalition defers questions pertaining to captured ISIS fighters to their relative nations' Departments of State or equivalent agencies. The Coalition's mission is to defeat ISIS in Syria and Iraq, and we will pursue ISIS fighters regardless of nationality.”

At a Thursday morning press conference, the chief spokesman for the U.S. military command waging the war against ISIS referred all questions about this story to the State Department. Asked if the “supposed U.S. citizen” was among five ISIS fighters who recently surrendered in Raqqa, Col. Ryan Dillon replied, “He was not.”

A State Department official said: “We are aware of that report. We have no information to share at this time.” The State Department official referred the Daily Beast to the Justice Department, which had already declined comment; the Justice Department reiterated that position Thursday morning.

For now, the unnamed U.S. citizen is in military custody as an enemy combatant. What the Trump administration does next – hold him in military detention, at Guantanamo Bay or elsewhere, or charge him in civilian court – will be a crucial test for how it handles wartime detentions. It was unclear late Thursday if the Trump administration had come to any decision, as it is confronting a problem of an American captured on an active battlefield for the first time since taking office.

But this American, whose name is not yet known, is not the first to be captured fighting for ISIS. In November, a federal court in Virginia indicted Mohamad Jamal Khweis, a U.S. citizen from the suburbs of Washington, D.C., on two counts of material support for terrorism. Khweis had surrendered to Kurdish forces in March 2016 and later expressed regret in a TV interview for traveling to Syria to join ISIS. And in July, Kurdish authorities released another video of a purported surrender of a U.S. citizen who fought for ISIS. The man was identified in local news stories as a Pakistani-American with the initials T.M., but no further information has been released.

What to do with American terrorist suspects captured abroad has been the subject of dispute between successive administrations, Congress, federal courts, military commands, and international bodies for 16 years.

The Bush administration initially claimed sweeping powers of detention. Louisiana-born Yaser Hamdi was captured in Afghanistan in 2001, transferred to Guantanamo in 2002 until his U.S. citizenship was confirmed, and was held for two more years in a different military facility before the Supreme Court ruled in a landmark 2004 case that Hamdi had the right to challenge his detention.

Although President Barack Obama preferred charging terror suspects in civilian courts, he took even more extreme measures when encountering U.S. citizens allied with al Qaeda overseas. Obama ordered the execution of Anwar al-Awlaki, an American propagandizing for al Qaeda in Yemen and inspiring terrorists in the United States, by drone strike in 2011.

As a presidential candidate, Donald Trump embraced brutal treatment of wartime detainees. He pledged “worse” torture than waterboarding, which is a violation of U.S. law, and to target the families of terror suspects, which is a war crime. Trump indicated in August 2016 his openness to detaining American citizens at Guantanamo—also currently illegal—and charging them with war crimes. Regardless of nationality, Trump told crowds of his eagerness to “load [Guantanamo] up with some bad dudes.”

Several of Trump’s senior administration officials have taken similarly hardline stances. Mike Pompeo, his CIA director and loyalist, told his Senate confirmation hearing that he opposed bringing back torture but softened his opposition in lesser-noticed written questions. One of the nine U.S. senators to vote against a 2005 law intended to ban the military from torture was Jeff Sessions, now the attorney general.

Jim Mattis, the defense secretary, apparently persuaded Trump in a December conversation to eschew torture. But whatever opposition Mattis has to torture does not extend to wartime detention. He told senators about an indefinite conflict: “Detention for the duration of hostilities to prevent a combatant’s return to the battlefield is a fundamental precept of the law of armed conflict. Long-term detention is appropriate when an unprivileged enemy belligerent poses a continuing significant threat to the security of the United States.”

But thus far, Trump’s detentions policies have adhered to the Obama-era status quo rather than departing from it. Trump has delayed issuing a long-anticipated executive order clearing the way for expanding the Guantanamo population from its current 41 detainees. In July, Sessions’ Justice Department charged a terror suspect, a non-U.S. citizen, in a Philadelphia federal court after Spanish officials turned the suspect over to U.S. custody. The exception to the consistency with Obama is Rex Tillerson’s decision to end the State Department office tasked with negotiating repatriations for Guantanamo detainees not facing war crimes charges.

Even under Obama—whose vow to close Guantanamo was thwarted by Congress, the Pentagon, and his own lassitude—the military improvised ways to detain terror suspects beyond Guantanamo. In the most famous case, U.S. special operations forces in 2011 held Ahmed Abdulkadir Warsame in the brig of the USS Boxer for months before Warsame was charged in a Manhattan federal court.

Warsame was captured in the waters off the Somali coast, which was not then a declared battlefield. Northeastern Syria is, meaning the military might assert the right to make battlefield detentions, a traditional wartime practice.

But doing so in Syria, where the U.S. holds no bases and is operating without international mandate in defiance of still-empowered dictator Bashar al-Assad, would immediately raise alarm about the abilities of the U.S. to safely detain someone and provide access to International Committee of the Red Cross monitors to verify proper treatment.

Should Trump stick with military detention, at a certain-but-unclear point, the military will either have to find a new location at an established base to hold the American captive or Trump will have to sign the executive order formally permitting an expansion of the Guantanamo detainee population—which will set off a wave of human-rights opposition and perhaps Democratic Party criticism.

Since the SDF has already relinquished custody of the American to the U.S., transferring to a different country is extremely unlikely.

The remaining option is to charge the American in a civilian federal court. The cases of hundreds of terrorism suspects, including those of Americans captured overseas, have been adjudicated through this option. The courts offer the benefits of both long-established legal procedure and a lack of international controversy, unlike military tribunals and indefinite military detention. Trump might, however, risk angering elements of his base, as the Republican Party has come to consider military outcomes to be the proper fate for suspected terrorists.

UPDATE: This story has been updated to including the U.S. military’s confirmation of The Daily Beast’s initial reporting.