(CNN) -- Seven U.S. Marines were killed in the midair collision of two U.S. military helicopters along the Arizona-California border, officials said Thursday.
Six killed in the crash were based at Camp Pendleton, California, and one was from Yuma, Arizona, the Marine Corps said.
The crash occurred during routine training operations Wednesday night around 10:30 p.m. ET, according to Marine Corps Air Station Yuma Base Commander Col. Robert Kuckuk.
The collision occurred in the Chocolate Mountains on the California side of the range, near Yuma.
The aircraft, an AH-1W Super Cobra attack helicopter and a UH-1Y Huey utility chopper, were part of the 3rd Marine Aircraft Wing and based at Camp Pendleton.
According to Gunnery Sgt. Dustin Dunk, a spokesperson for Air Station Yuma, two other helicopters were in the immediate area of the collision.
Authorities were investigating.
"We won't know exactly what happened until the investigation is complete, and we can't make any assumptions right now," said 1st Lt. Maureen Dooley, a Marine Corps spokeswoman.
While weather did not appear to play a factor, visibility may have been severely limited by nightfall and a significant amount of dust, referred to as a "brown out," created by the helicopters landing and taking off, according to Dunk.
The Marines on board were preparing for deployment to Afghanistan, Dooley said.
"We have aircraft going out to Yuma training ranges on a weekly basis, and they go out there primarily because the aircraft mimics what they will see in Afghanistan," Dooley said.
Authorities did not plan to release the names of the Marines involved until their families were notified.
"This tragedy serves as another stark and sad reminder of the peril our men and women in uniform encounter on a daily basis -- not only abroad, but on our own soil. It also reminds us that, whether in combat or training, no military mission is ever routine," Arizona Gov. Jan Brewer said in a statement. "We must never take for granted nor forget these soldiers' sacrifice and service to the United States of America."
Marine Corps Air Station Yuma is home to about 4,000 active duty Marines and sailors. It supports 80% of the Marine Corps air-to-ground aviation training and covers five square miles in southeastern Yuma.
The military has long used the AH-1W and UH-1 Huey for a variety of tasks. The military describes the AH-1W as "the backbone of the United States Marine Corps' attack helicopter fleet." The UH-1Y is a twin-engine, medium-size helicopter.
CNN's Stella Chan, Ashley Hayes, Moni Basu and Miguel Marquez contributed to this report.