The Houston native used the alias Abu Mohammad al-Ameriki. Sunday's SDF statement named the other American as Zaid Abed al-Hamid, 35, but did not list his hometown. He was also known as Abu Zaid al-Ameriki.
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NBC News reported last year that Clark's resume was found at a house in Iraq and was later obtained by the Program on Extremism at George Washington University. In a cover letter, Clark said he was hoping to obtain a job teaching English to students in territory seized by ISIS.
The resume included an email address, education credentials and work experience, and the researchers were able to determine that Abu Muhammad al-Ameriki was Clark, who graduated from the University of Houston.
The men were caught alongside two other foreign militants from Pakistan and a fifth from Dublin, Ireland, according to the Kurdish-led SDF. The alleged cell was said to have been attempting to attack civilians as they fled former ISIS strongholds.
U.S. officials were not immediately able to confirm that the Americans had been detained.
Shiraz Maher, deputy director of the International Centre for the Study of Radicalization and Political Violence at King's College London, said there were conflicting reports on how many foreign fighters remained in Syria.
“Some have been captured, some have been killed and some are still alive and operational,” he said.
Last year, a report by George Washington University’s Program on Extremism found that 64 Americans had traveled to Iraq and Syria in support of the Islamic State since 2011. This compares to the thousands of Europeans who joined ISIS during the same period.
Saphora Smith is a London-based reporter for NBC News Digital.