Three U.S. Marines found unresponsive in a car at a North Carolina gas station died of carbon monoxide poisoning, the local sheriff’s office said Wednesday.

Deputies from the Pender County Sheriff’s Office found the three men Sunday morning in a privately owned Lexus sedan parked outside a Speedway gas station in the coastal community of Hampstead. Autopsies performed Wednesday by the North Carolina medical examiner’s office determined that all three deaths were the result of carbon monoxide poisoning, according to the sheriff’s office.

Sgt. Chester Ward of the Pender County Sheriff’s Office told The Associated Press Wednesday that the situation “seems accidental.” Investigators “have found nothing else that shows that it’s something else,” such as suicide.

The three lance corporals, identified by the U.S. Marine Corps as Tanner J. Kaltenberg, 19, of Madison, Wisconsin, Merax C. Dockery, 23, of Pottawatomie, Oklahoma, and Ivan R. Garcia, 23, of Naples, Florida, were stationed at nearby Camp Lejeune, located 29 miles (47 kilometers) northeast of the gas station. They were motor vehicle operators with the Combat Logistics Battalion 2, Combat Logistics Regiment 2 and 2nd Marine Logistics Group.

“I am saddened by the timeless and tragic death of these three young men, who served our country honorably,” Sheriff Alan Cutler said. “Our thoughts and prayers remain with their families and colleagues during this time.”

Ward said the department had received a missing person report early Sunday morning from the mother of one of the Marines after her son failed to arrive on a flight home the night prior.

Dockery’s mother, Heather Glass of Maud, Oklahoma, said Wednesday that she and another relative had driven to the Oklahoma City airport last Saturday evening to wait for her son to fly home for his grandfather’s funeral.

When he didn’t arrive, Glass’ daughter started calling North Carolina hospitals and jails while Glass contacted the sheriff’s office and her son’s sergeant at Camp Lejeune, resulting in a search.

Glass said she assumed that her son died from something like carbon monoxide because all three of the young Marines had died. Breathing too much carbon monoxide makes victims pass out. Ward had said Tuesday before the autopsy that the sheriff’s office did not suspect foul play.

“I was just worried that it was something worse,” Glass told the AP in a phone interview.

“I’m at peace. I feel at peace because I know he was asleep when he passed,” Glass said.

Dockery was the youngest of five siblings — the rest of them older sisters — and grew up in nearby Seminole. Glass said her son joined the Marines “for personal growth” and so that he could travel, with the possibility of making the military a career.

Glass said funeral arrangements were being assembled, with dates based on where her son’s body can be released to the family.

“He was just a kind soul,” Glass said. “He was liked by everybody. He was a real good kid.”