The Syrian Kurdish YPG militia on Wednesday reported the capture of eight more foreigners fighting for the Islamic State, including a 16-year-old American identified as Soulay Noah Su.

As with the operation that captured two American ISIS fighters this week, the YPG stated it was forced to attack an Islamic State cell in the Deir ez-Zor area because it was “planning attacks to slow down the progress of our forces by attacking civilians as a last resort.”

“Eight Daesh terrorists of American, Russian, Ukrainian, German, Kazakh, Tajik and Uzbek origin were captured during the operations carried out by our Special Operations Team on January 6th and 7th,” the Kurdish militia announced. “Daesh” is another name for ISIS.

According to the statement, Soulay Noah Su also used the name “Abu Souleiman al-Amriki” while serving the Islamic State. No other information beyond his age and nationality was provided.

The U.S. State Department indicated on Wednesday that it was aware of the Kurdish report but did not confirm the capture. Most of the countries with citizens allegedly captured by the Kurds while fighting for ISIS have approached the reports with some delicacy; the Ukrainians seem inclined to dismiss them as outright propaganda spread by Russia to make Ukraine appear sympathetic to terrorists, or at least highly debatable claims of Ukrainian nationality for the detainees.

This is partly due to the problems associated with prosecuting and repatriating foreign ISIS fighters, coupled with Kurdish complaints that holding them for much longer will be difficult, as summarized by CBS News on Wednesday:

The U.S. has been working with allies for months to figure out what to do with all the suspected ISIS militants in Kurdish custody. As CBS News Radio correspondent Cami McCormick reported in December, then-Defense Secretary James Mattis tried unsuccessfully to convince countries to take back their nationals. Part of the problem for the ISIS fighters’ home countries are the legal ramifications of holding citizens who were picked up on the battlefield — and how to prosecute them. “It’s one thing for the government to be very confident that an individual joined or tried to join ISIS,” Joshua Geltzer, a senior counterterrorism official in the Obama administration, told the Associated Press this week. “Sometimes it’s still another thing for the government to be able to mount confidently a criminal prosecution against that individual.” The Kurds warned last month that if U.S. forces withdraw from Syria, they might have to turn their attention to fending off an attack from neighboring Turkey, which considers the YPG a terrorist organization, and in turn release the prisoners. The fear is not only that those foreign fighters could help ISIS rebuild in Syria and Iraq without a U.S. military presence, but that they could return to their own countries and launch attacks.

The Lawfare blog explained how Soulay Noah Su’s case could pose additional legal problems due to his age and the way he was captured:

Su joins at least five other American travelers who have been captured on the battlefield in Syria and Iraq. In the majority of these cases, fighters were captured by sub-state actors—the YPG, SDF and forces aligned with the Kurdish Regional Government in northern Iraq. This adds complications to efforts by the Department of Justice to return Americans to the U.S. to face trial. In these cases, law enforcement must take additional steps to secure evidence, ensure consular rights of U.S. citizens arrested abroad, and negotiate their extradition. In certain cases, the U.S. government successfully gained custody of accused Islamic State members from detention by Syrian and Iraqi militia groups. Last year, the Justice Department indicted two U.S. citizens, Ibraheem Musaibli and Samantha Elhassani, in the Eastern District of Michigan and Northern District of Indiana respectively, after securing their transfer to U.S. custody from the SDF. In another well-publicized case, however, the suspect in question never saw a courtroom: Abdulrahman Ahmad Alsheikh, better known as John Doe, was held in U.S. military custody after his capture by the SDF, and was transferred to Bahrain following a lengthy court battle. On top of the array of legal issues that would be present in any such case, perhaps the more pertinent question here is Su’s status as a minor. Our research at the Program on Extremism found 14 American minors in jihadist-controlled territory in Syria and Iraq, the majority of whom are young children who traveled along with relatives who eventually joined jihadist groups. From that number, seven were returned to the United States. Due to their young age and other mitigating factors, no minors who returned to the U.S. have been prosecuted.

Lawfare noted that Su would be the only American minor who participated in battlefield operations for the Islamic State, which could make stringent penalties difficult to apply, while law enforcement and national security officials would be understandably nervous about lighter approaches such as counseling or return to family custody.