Fort Campbell crash leaves 9 service members dead after 2 US Black Hawks helicopters collide in deadly training session

NINE service members have died following an aerial collision involving two US Black Hawk helicopters.

The deadly training session occurred near the Fort Campbell military base in Trigg County, Kentucky - near the Tennessee border about 60 miles northwest of Nashville.

The two Black Hawk helicopters were from the 101st Airborne Division, Army officials confirmed.

They were flying in a "multi-ship formation under night vision goggles" when the crash occurred, officials said.

Officials have confirmed that they have started the process of notifying the victims' families.

Kentucky Governor Andy Beshear said it's a "tough and tragic day" for Kentucky, Fort Campbell, and the 101st Airborne Division.

He added: "We are blessed to live in the freest country on planet earth, but must remember that freedom relies on those willing to serve."

Senate Minority Leader Mitch McConnell said: "I am devastated to learn about the Army helicopter accident over Kentucky involving our brave 101st Airborne."

First responders raced to the scene after the helicopters crashed to the ground.

An eyewitness told WKDZ that he heard "a pop" and "two booms" from his home, located a half mile away from the crash site.

And, Nick Tomaszewski told the NBC affiliate KYTV: "There were two that were coming kind of straight up over our house, headed straight northbound.

"I told my wife, ‘Wow, those look really close tonight,’ for whatever reason."

Nick said there was an "explosion" in the sky that looked like a firework.

There was light to no wind in the area at the time of the crash.

Officials have not revealed what caused the helicopter crash amid the ongoing investigation.

Several helicopters had been flying across the area in recent nights, reports say.

Black Hawk HH-60s are versatile combat service helicopters. 

The twin-engined crafts are used to conduct medical evacuations, air assault missions as well as special operations, according to the Army.

An HH-60G Pave Hawk helicopter can reach a maximum speed of 223 miles per hour and can carry between eight to 12 troops.

The crash comes just a month after a UH-60 Black Hawk helicopter slammed onto a highway in Alabama.

Two crew members from the Tennessee National Guard were killed in the incident.