Nine US soldiers were killed when two US Army helicopters crashed in Kentucky during a training practice late Wednesday in southwestern Kentucky, officials said.

The crash involving the HH-60 Black Hawk helicopters from the 101st Airborne Division occurred in Trigg County, Kentucky, west of the army post Fort Campbell on Wednesday night at around 10 p.m. local time (11 p.m. ET) while all nine troops were wearing night-vision goggles, according to a statement from the division.

It is not clear what caused the crash, but an investigation is currently being conducted to determine what happened.

At a news conference on Thursday, Brig. Gen. John Lubas, the division’s deputy commander, said that there were five people on board one of the aircraft and four people on the other. Black Hawks can carry up to 12 people and are normally flown by a pilot, a co-pilot, and a crew chief.

The helicopters were used for medical evacuation, and it’s thought the disaster occurred while they were in the air, not during a medical evacuation drill.

The general said that an aviation investigations team from Fort Rucker in Alabama has been deployed to investigate the possibility that the helicopters collided.

“At this point, we don’t know. We’re hopeful that when we get the team from Fort Rucker here and they’re able to pull some of the data out of the onboard computers we will have a better understanding of exactly what happened,” he said

According to Staff Sgt. Joshua Tverberg, spokesman for the 101st Airborne Division, four helicopters were participating in the training exercise, two of which were the Black Hawks that crashed. Among the helicopters, one had stopped to refuel while another was flying ahead of the two that eventually crashed.

The names of the deceased will not be made public until their families have been notified, Gen. Lubas said.

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​‘A heavy day for the Army’

Army Secretary Christine Wormuth said Thursday that it was “a heavy day for the Army,” and thanked lawmakers for their condolences during a Senate hearing in Washington, DC.

“Thank you for your comments and thoughts and prayers for the families of our soldiers who were killed in the crash,” Wormuth said. “Our hearts go out to them.”

“I am devastated to learn about the Army helicopter accident over Kentucky involving our brave 101st Airborne,” Senate Minority Leader Mitch McConnell of Kentucky said in a statement on Twitter. “My team is in contact with the Army and authorities on the ground. Please pray for our service members and their families as we learn more.”

Defense Secretary Lloyd Austin, in a statement, praised those “who bravely and proudly serve our country each and every day.” He called the incident “tragic” and said, “I am working with Army leadership to make sure our troops and their families receive the care that they need in the wake of this accident.”

At the news conference on Thursday morning, Kentucky Governor Andy Beshear (D) joined Lubas in expressing how much their communities and families would miss the fallen soldiers.

“We are blessed to live in the freest country in the history of planet Earth, but we must remember … that freedom relies on those who are willing to serve, some of which paid the ultimate price,” Beshear said.

Less than two months before this crash, two Tennessee National Guardsmen lost their lives in Alabama when the UH-60 Black Hawk they were flying during a training exercise crashed.

According to the US Army Combat Readiness Center, the Army has experienced an average of five deaths each year in aviation accidents during on-duty activities since the 2018 fiscal year.