Seven US Marines were killed when two helicopters collided during night training exercises near the border of California and Arizona, the Marine Corps said on Thursday.
The aircraft collided late yesterday in a remote portion of the Yuma Training Range Complex.
"We're still gathering a lot of details as the sun comes up," said Lt. Maureen Dooley with the Miramar Air Base in California.
It will be at least 24 hours before the Marine Corps releases the names of those killed.
Dooley told CNN that helicopter teams train in the area because it mimics conditions in Afghanistan.
When asked whether the crash was a result of technical or pilot error, she said she couldn't confirm anything because the investigation had just begun and could take months.
Several accidents have happened in the past year involving Marine Corps training in Southern California.
In September, a helicopter went down at Camp Pendleton north of San Diego, killing the two Marines on board.
In August, two Marines were ejected from their fighter jet as it plunged toward the Pacific Ocean. They spent four hours in the dark, chilly ocean before they were rescued. Both had broken bones but survived.
In July, a Marine was killed when his helicopter went down in a remote section of Camp Pendleton.
Another fighter jet sustained at least USD 1 million damage when its engine caught fire on March 30 aboard the USS John C Stennis during a training exercise off the San Diego coast. Eight sailors, a Marine and two civilians were injured.