SUGAR LAND — We’re learning more about a former Fort Bend ISD substitute teacher who allegedly went to work for ISIS.

Warren Christopher Clark, 34, was reportedly captured by Kurdish rebels over the weekend.

His alleged ties to ISIS surfaced when multiple news organizations, including the New York Times, cited documents he reportedly sent to the terror group that were later found in a house in Mosul, Iraq.

Clark, who also goes by Abu Muhammad, reportedly sent his resume and a cover letter to ISIS seeking a teaching position.

Clark was a substitute teacher for Fort Bend ISD as recently as 2010.

A 2008 graduate of the University of Houston, Clark moved to the Middle East to teach English, the New York times reported.

Clark's father, who still lives in Sugar Land, told the Times his son is a "humanitarian" and the allegations aren't true.

“My son would not be involved in anything along those lines,” Clark said. "My son doesn’t have an evil thought in his mind about hurting anyone."

Sam Chenh, a neighbor, said he was "very surprised" by the reports. Chenh recognized Clark in a photo released by the rebels.

“It’s a good family,” said Chenh. “We’re good neighbors here.”

Georgetown University professor Seamus Hughes tracks Americans who join ISIS and first uncovered the documents tied to Clark, including a cover letter. He said that’s something he’d never seen.

“I’ve seen a lot of things in my days looking at terrorism,” said Hughes. “And I’ve never seen something like that.”

Hughes said it’s extremely rare for Americans to join ISIS. Only about 60 or 70 have made it over to the Middle East.

It's unclear when or if Clark will be brought back to the U.S. to face possible charges. We checked with the Department of Defense which cannot confirm Clark’s capture and said the matter is under investigation.