Story highlights Terence Hildner, 49, died of "apparent natural causes"

He was commander of the 13th Expeditionary Sustainment Command

Hildner was in Afghanistan to support the NATO training mission

His death is under investigation

U.S. Brig. Gen. Terence Hildner died in Kabul of apparent natural causes, officials said, making him the highest ranking officer to die in Afghanistan.

He was 49.

Hildner, who was commander of the 13th Expeditionary Sustainment Command, Fort Hood, Texas, was in Afghanistan to support the NATO training mission.

"The unfortunate and untimely death of Brigadier General Hildner was a shock to our unit and Families," Col. Knowles Atchison, 13th ESC rear commander, said in a statement posted on the Fort Hood website. "Both forward deployed elements and we at home station are deeply saddened by this loss. We will all pull together through this difficult period and care for one another."

The circumstances surrounding his death are under investigation, the statement said.

Hildner graduated from the University of Notre Dame in 1984 and attended the U.S. Army Command and General Staff College in 1997. A Connecticut native, he listed his home in Fairfax, Virginia.

In 2003, Hildner assumed command of the 13th Corps Support Command's Special Troops Battalion at Fort Hood. That battalion conducted two operational deployments during his years in command -- the first in support of Operation Iraqi Freedom, the second in the wake of Hurricane Katrina.

Previously, Hildner served in Iraq during Operation Desert Storm and participated in the last U.S. patrol along the East-West German border before its reunification.