A 34-year-old man from Houston who is said to have sent a résumé and cover letter seeking a job with the Islamic State has been seized on a battlefield in Syria, an American-backed militia fighting the militants said Sunday.

“Dear Director, I am looking to get a position teaching English to students in the Islamic State,” the Texan, Warren Christopher Clark, is said to have written in a letter found in an Iraqi house once occupied by the militants.

Mr. Clark once worked as a substitute teacher in the Fort Bend Independent School District in Sugar Land, Tex., according to his father, and seemed prepared to draw on that experience.

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