An Americans captured by The Syrian Democratic Forces — a U.S.-backed militia fighting the Islamic State — had applied for a job teaching English to students of the Islamic State.

“Dear Director, I am looking to get a position teaching English to students in the Islamic State,” Warren Christopher Clark, 34, wrote in a letter to ISIS, according to the New York Times.

“I believe that a successful teacher can understand a student’s strengths and weaknesses,” he said, adding that teachers should “use that understanding to help students build on their understanding of the English language.”

Clark, 34, is originally from Houston and previously served as a substitute teacher in the Fort Bend Independent School District in Sugar Land, Texas after graduating from the University of Houston. Documents recovered from a home in Mosul, Iraq indicate that Clark also had taught English in Saudi Arabia and Turkey. Positions after 2015 are vacant on his resume and suggest he joined ISIS afterwards.

His father denied that he was part of ISIS, and claimed his “son would not be involved in anything along those lines.” “My son doesn’t have an evil thought in his mind about hurting anyone,” said Warren Clark.

The Syrian Democratic Forces announced Sunday that Clark had been captured with another American, Zaid Abed al-Hamid, along with others from Pakistan and Ireland. The Pentagon said the incident is under investigation.

“We are aware of open source reports of reportedly American citizens currently in custody who were believed to be fighting for ISIS," Pentagon spokesperson Cmdr. Sean Robertson said in a statement. "However, we are unable to confirm this information at this time. The incident is under investigation."