The U.S. Army said Friday it is temporarily halting Apache helicopter training flights after two soldiers were killed earlier this week in a crash near Fort Hood, Texas.

“The stand-down will remain in effect until we have a better understanding of the root cause of the accident,” the Army said in a statement.

The AH-64 Apache attack helicopter crashed Wednesday in a field in rural Salado, Texas, near its base in Fort Hood, the Army and local law enforcement officials said. The Army identified the deceased pilots as Chief Warrant Officer 2 Deontre T. Huey, 34, and Warrant Officer Seth L. Olmsted, 25, both Texas natives.

“You could tell this was a violent crash,” said Bell County Sheriff’s Office spokesperson Bill Coleman, who told reporters the helicopter did not hit any buildings, but sparked a wildfire.

The military has not specified the cause of the crash. It is not unusual for the military to order a temporary stand-down after a fatal crash, especially if the cause is still under investigation.

“The Army is profoundly saddened by the loss of two Soldiers in the recent AH64 Apache helicopter crash near Fort Hood, Texas,” the Army said Friday. “We mourn these aviators, and our deepest condolences remain with their families, friends, and unit during this extraordinarily difficult time.”

The Army has described the newest iteration of the AH-64 Apache, which has been manufactured by Boeing since the late 1990s, as “the most advanced aerial fighting vehicle in the world.” The twin-engine helicopter typically seats a pilot and a co-pilot gunner, and is equipped with a rapid fire gun and can launch Hellfire missiles.

Hundreds of Apaches are operated by Army and National Guard units, and the helicopters have been used during the U.S.-Iran war. In June, an Apache that was patrolling the Strait of Hormuz was shot down by an Iranian drone, President Trump announced. The two pilots were then rescued by a sea drone.