Nine soldiers were killed after two United States Army helicopters collided during a training mission near an Army base along the Kentucky-Tennessee border on Wednesday night, the Army said.

The two HH-60 Black Hawk assault helicopters crashed into each other at about 10 p.m. during a routine training mission in Trigg County, Ky., Nondice L. Thurman, a spokeswoman at the Army base, Fort Campbell, said in a statement. She added that the crash was under investigation.

The helicopters were from the 101st Airborne Division, which is based at Fort Campbell and is the Army’s only air assault division. The division said on Twitter early Thursday that the accident had resulted in several casualties, but it did not elaborate. “Right now our focus is on the soldiers and their families,” it said. Later, another Army spokeswoman, Dawn Grimes, confirmed that all soldiers who were on board had died.

Gov. Andy Beshear of Kentucky had said earlier on Twitter that the state’s police and emergency management agency were responding to the accident. He said he was traveling to Fort Campbell to support troops and their families.

A Black Hawk helicopter can transport an 11-person infantry squad, and the HH-60 model can be used for air assaults, medical evacuations and other purposes, according to the Army.

In 2018, seven servicemembers died when another type of U.S. military helicopter, an HH-60G Pave Hawk, crashed in Iraq. A military investigation later found that, as a result of a pilot error, the helicopter had struck a steel cable strung horizontally between two buildings.

The weather in the Fort Campbell area early Thursday morning was fair: calm winds, a visibility of 10 miles and a temperature of 39 degrees.

Fort Campbell sits on 105,000 acres that include parts of Trigg and Christian Counties in Kentucky, and Montgomery and Stewart Counties in Tennessee.

This is a developing story.