WASHINGTON — A man believed to be an American citizen who was fighting with Islamic State militants has surrendered in Syria and is now in U.S. custody, raising questions about how the Trump administration will deal with such detainee cases.

Maj. Adrian Rankine-Galloway, a Pentagon spokesman, said the man surrendered to Syrian Democratic forces around Tuesday and was turned over to U.S. military force in Syria. Rankine-Galloway said the man is being legally detained as a known enemy combatant.

The decision to legally detain the man as a “known enemy combatant” comes as the Trump administration has been working to craft a detention policy, which could determine whether the U.S. will resume sending detainees to the military prison at Guantanamo Bay, Cuba. President Barack Obama did not send any new detainees to Guantanamo and the currently there are only about 40 there. Trump has expressed a willingness to send new detainees there, but so far has not.

It’s not clear if the detainee, who has not been identified, and the U.S. forces holding him are still in Syria, but the plan is to take the man into Iraq, where he would be then turned over to the U.S. State Department or Justice Department.

U.S. officials said they are working out what they will do with the man, if he is confirmed as an American. If his citizenship is confirmed, he would be the second known American who was taken into custody for fighting with IS insurgents.

Trump administration officials have said that there are three viable options today for taking enemy combatants off the battlefield: They can be killed; they can be apprehended and released after a few weeks, which could involve interrogation; or they can be captured and handed over to a third party. The third approach causes concern about the possibility that detainees outsourced to a third party could be treated inhumanely. Suspects accused of terror-related offenses, including Americans, also can be adjudicated in U.S. courts.

A Virginia man who joined IS in Syria and Iraq for several months surrendered to Iraqi Kurdish forces in northern Iraq in March 2016 and was flown back to the U.S. to face charges. A jury in a federal court in Virginia took only four hours to convict Mohamad Khweis, 27, of Alexandria, Virginia on terrorism charges.

Khweis could potentially face 20 years or more in prison when he is sentenced in October. His lawyers acknowledged that Khweis left his home in Virginia in December 2015 to join IS militants, but argued that didn’t make him a terrorist; the defendant said he was only there to “check things out.”

Pardiss Kebriaei, a senior staff attorney for the Center for Constitutional Rights, said that even if the detainee was directly fighting in the war he must be “charged promptly or released, not held indefinitely, and treated humanely in the interim.”

As an American, Kebriaei said, “this person retains his constitutional rights, and he must be dealt with through the civilian criminal justice system.”