(CNN) -- Two crewmen aboard an Idaho Army National Guard helicopter died when it crashed Thursday night, according to Ada County Coroner Erwin Sonnenberg. Guard spokesman Col. Timothy Marsano said the Apache helicopter was on a training mission when it went down two miles south of the Boise Airport. "It's a very difficult evening for us in the Idaho National Guard," Marsano told CNN affiliate KBOI. "We don't know the reasons why. We can't speculate at this point." The military is investigating the crash site. "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 told the affiliate. "Today, our thoughts and prayers are with their families." CNN's Greg Morrison contributed to this report .
The Idaho Army National Guard helicopter crashed near Boise Airport . The Apache helicopter was on a training mission . An investigation is underway .