Nine soldiers were killed in a crash of two medical evacuation Black Hawk helicopters during a routine nighttime training mission over Kentucky, the US Army said on Thursday, in one of the military’s deadliest training accidents in recent years.

Crew members using night-vision goggles were flying two HH-60 Black Hawk helicopters, operated by the Army’s 101st Airborne Division, when the aircraft crashed in a field late on Wednesday over Kentucky’s Trigg County.

Army Brigadier General John Lubas, the division’s deputy commanding officer for operations, said little was known so far about why the helicopters came down and he acknowledged he was not even sure whether they crashed into each other.

Lubas said an aircraft safety investigations team from Fort Rucker in Alabama would arrive later on Thursday to review data from onboard computers, which he said were similar to the black boxes in commercial aircraft.

“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,” Lubas told reporters outside the base.

Lubas added that one helicopter had five soldiers aboard while the other had four, and they were using night-vision goggles.

The HH-60 is a variant of the Black Hawk helicopter designed to provide support for various military operations, including air assaults and medical evacuations, according to the Army.

Army Secretary Christine Wormuth addressed a Senate committee, where lawmakers expressed their condolences, and said it was tough moment for the US military’s biggest service branch.

“Thank you for your comments and thoughts and prayers for the families of our soldiers who were killed in the crash. Our hearts go out to them,” Wormuth told the Senate Armed Services Committee. “It’s a heavy day for the Army.”

The weather in Fort Campbell area, which straddles the border between Kentucky and Tennessee, featured clear skies and calm winds on Wednesday night, according to National Weather Service forecaster Marc Chenard.

Fort Campbell, home to the 101st Airborne Division, is one of the largest military bases in the United States. Two US soldiers were killed in a helicopter crash during training at Fort Campbell in 2018.