DICKENS, Tex.

PAUL SLOUGH may have worked as a cowboy growing up in this tiny town in northwest Texas, but soldiers who served with him were stunned to hear he had been accused of acting like one as a Blackwater security guard in Iraq.

“I went on 20 to 30 missions with Paul. You could always depend on him,” said Jeremiah Thompson, recalling his tour of duty with Mr. Slough in Iraq for the Texas National Guard. “He was always careful. He was always professional. I never knew him to break the rules of engagement.”

Today, Mr. Slough, 28, is at the center of a federal investigation into the Sept. 16 shooting deaths of 17 Iraqis in Baghdad by a convoy of Blackwater security guards. Authorities have refused to talk about the inquiry, except to say it has focused on one guard, identified only as “turret gunner No. 3.”

Through a review of case documents and interviews in Texas and Washington, The New York Times identified the gunner as Mr. Slough, a former infantry soldier who joined Blackwater Worldwide after his dreams of joining the Army Special Forces were quashed by recurring problems from an old football injury.