Two members of the Idaho Army National Guard were killed when their helicopter crashed Thursday, CNN reports. The Apache helicopter crashed several miles south of the Boise Airport.

“The men and women of the Idaho National Guard are united in grief at the loss of two of our brothers in arms, who gave their lives while training to defend our nation,” Idaho National Guard Cmdr. Maj. Gen. Gary Sayler said in a statement obtained by ABC News. “Today, our thoughts and prayers are with their families.”

ABC News adds that the Idaho National Guard does not know why the helicopter crashed, but that the U.S. Department of Defense is investigating.

The names of the two victims, which are both male, will not be released until their families have been notified, KTVB.com reports. The Ada County Coroner’s Office will be conducting the autopsies.

According to Boeing, the AH-64 Apache is the most advanced multi-role combat helicopter for the U.S. Army. The company says that in all, 14 nations fly, have ordered or have selected the AH-64 Apache helicopters for their defense forces.