Two American citizens fighting for ISIS have been apprehended in Syria after Syrian Democratic Forces (SDF) special operations units raided an Islamic State compound near the Syria-Iraq border on Sunday. Warren Christopher Clark, a 34-year-old originally from Texas who goes by the name Abu Muhammad al-Ameriki, was arrested along with another American expat—Zaid Abed al-Hamed, 35—after U.S.-backed forces discovered the militant group was preparing to launch terrorist attacks against Syrian civilians in the area. In addition to the two Americans, the SDF also captured two fighters from Pakistan and one from Ireland.

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“The incident is under investigation,” said U.S. Army Col. Scott Rawlinson, spokesman for Combined Joint Task Force Operation Inherent Resolve (CJTF-OIR), according to a report from Stars and Stripes.

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It appears the two Americans may have traveled to Syria in order to work for the Islamic State in a “noncombat” capacity. Originally, Clark wanted to teach English at a school in the Islamic State’s self-proclaimed capital of Mosul. Clark is a University of Houston graduate and worked as a substitute teacher for a short while before leaving the U.S. It is believed he converted to Islam around 2004, and was self-radicalized by jihadi-propaganda videos on the internet. The information on Clark was gathered by George Washington University researchers working with the school’s extremism program. The program has identified as many as 64 Americans who have fought for ISIS since 2011. Zaid Abed al-Hamed, the other American captured by the SDF, was not known to the researchers before his arrest.

The raid took place in the city of Deir ez-Zor, which has been the scene of intense violence since the beginning of the Syrian Civil War in 2011. The Syrian Defense Force believes that the city is “the last stronghold of ISIS in Syria.” Fighting has been especially intense in Deir ez-Zor in recent days, and as a result, much of the city’s civilian population is fleeing for safety. The SDF believes that the five ISIS terrorists captured were planning on exploiting the situation, launching attacks on fleeing civilians.

“Heavy clashes are taking place currently between our forces and ISIS terrorists due to an attack by ISIS on one of our points in Dier ez-Zor,” wrote Mustafa Bali, a press officer with the SDF, on Twitter a day before the raid. “Everybody should be aware that #ISIS is not defeated.”