SAN DIEGO – Six of 18 Marines injured in a rollover accident were listed in critical condition Friday, a day after the multi-ton truck carrying them flipped on a paved two-lane road 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. All 19 Marines were from the 1st Marine Division, 1st Lt. Colleen McFadden said.

The name of the one Marine killed 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 Basilone Road, one of the main roads at the vast coastal base dotted with mountains. No other vehicles were involved in the accident, McFadden said.

The troops were returning from routine training at the time, McFadden said.

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 truck the Marines were traveling in is considered to be a workhorse for the Navy and Marine Corps, carrying troops, supplies, and heavy equipment. Called a Medium Tactical Vehicle Replacement or MTVR, it is built to go over any terrain and operate in weather conditions ranging from minus 50 degrees Fahrenheit to 125 degrees Fahrenheit. Despite its top heavy appearance, the hulking vehicle can ford 5 feet of water, climb a 60-percent gradient and navigate a 30-degree side slope, according to its manufacturer Wisconsin-based Oshkosh Corp. It can carry 15 tons over the highway and up to 7 tons off-road.

Oshkosh officials declined to say whether there have been any other rollovers involving the MTVR and referred all questions to the Marine Corps.

The Marine Corps said it had no further information 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.