SAN DIEGO (AP) — Six of 18 Marines injured in a rollover accident were listed in critical condition Friday, a day after the 7-ton truck carrying them flipped during routine training at Camp Pendleton, killing one Marine.

Eight of the troops were in stable condition and four were treated and discharged from medical facilities in San Diego County, according to the Marine Corps. The Marine who was killed was from the 1st Marine Division, and that person’s name will not be released until 24 hours after relatives are notified, officials said.

It’s unclear whether the truck — commonly used to transport Marines and haul supplies — hit something, had a mechanical failure or was traveling at a high rate of speed late Thursday afternoon on the vast coastal base dotted with mountains. Authorities said the investigation into the cause of the accident could take months.

“After this tragic training accident, our first concern is for the welfare of our deceased Marine, the injured and their families,” Maj. General Daniel J. O’Donohue, commanding general of 1st Marine Division, said in a statement Friday. “They are held close in our hearts and prayers. Our commanders, chaplains, medical personnel and families and surrounding communities have come together in support. We deeply appreciate all the support and sympathy that we have received for this tragic loss.”

The Marine Corps said it had no other details to release at this time.

Camp Pendleton is the major West Coast outpost for the U.S. Marine Corps.

In November 2013, four Marines were killed there while clearing explosives in a Camp Pendleton training area.