Nine US troops died after two US Army helicopters crashed during a training mission in Kentucky, officials say.
The Wednesday night crash involved the 101st Airborne Division and happened near the Fort Campbell military base.
It took place at 22:00 (02:00GMT) while all nine troops were wearing night-vision goggles, said a top US general.
It is unclear what caused the crash and an investigation into what happened is under way. No injuries were reported of anyone on the ground.
Two HH60 Blackhawk helicopters crashed during “a routine training mission,” an Army spokesman told BBC News on Thursday.
Army Brigadier General John Lubas told reporters that one chopper was carrying five people while the other carried four in what he described as a “fairly typical” arrangement.
The group had been practicing a “multi-ship formation” and medical evacuation drills, he said.
But he added that the crash appears to have occurred while they were flying, and not while in formation.
The force is “doing everything we can” to notify next-of-kin, Gen Lubas said, adding that some of them are outside of the US.
Speaking at a Senate hearing in Washington DC, Army Secretary Christine Wormuth thanked lawmakers for their condolences.
“It’s a heavy day for the Army,” she said.
Weather on Wednesday night in the area – approximately 60 miles (100km) northwest of Nashville – was calm and clear.
An air investigations team from Fort Rucker in Alabama has been deployed, the general said, and will examine the possibility that the helicopters collided. (BBC)