OKLAHOMA CITY (AP) -- A 29-year-old Army Reserve sergeant from Oklahoma was one of the seven American soldiers killed in an explosion at a weapons cache in Afghanistan, his sister said Friday.

Military officers notified relatives of Sgt. Kyle Seitsinger, a student at Oklahoma Christian University in Edmond, that he had died in the incident, his sister Penny Cockerell said.

"It makes it even worse that it was a training exercise. It makes it more senseless," Cockerell, a reporter who works in the Dallas bureau of The Associated Press, said from her family's home in Oklahoma City.

Seitsinger, who was attached to the Army's 10th Mountain Division out of Fort Drum, N.Y., was involved in efforts to find weapons caches along the border of Afghanistan and Pakistan. He was attending the university when he was called to active duty on Dec. 1.

"There are so many loose weapons here and the wrong guys maintain control of them in clandestine areas," he said in a Jan. 14 e-mail to his sister.

"Danger does follow our path, but I'll be all right."

Cockerell said her brother was adventurous and wanted to be a war correspondent one day.

He served in the Marine Corps after high school and was assigned duty at U.S. embassies in Moscow and in Brazil.

Survivors also include another sister, Karla Seitsinger, his father, Dan Seitsinger, and stepmother, Jo Seitsinger.