RALEIGH, N.C. (AP) — The seemingly accidental deaths of three U.S. Marines who suffered carbon monoxide poisoning in a parked car at a North Carolina gas station have raised questions about how the situation could have occurred outdoors.

Deputies from the Pender County Sheriff’s Office had found the men unresponsive in a privately owned Lexus sedan in the coastal community of Hampstead. Autopsies performed last week by the North Carolina medical examiner’s office determined that all three died of carbon monoxide poisoning. Sgt. Chester Ward from the sheriff’s office said the ongoing investigation indicates it was accidental.

While the Centers for Disease Control and Prevention report that many U.S. carbon monoxide deaths occur inside homes or closed garages, automotive experts say certain vehicle malfunctions can cause casualties outdoors.

Usually, those malfunctions are loud or smelly. If a car’s exhaust system is broken or is leaking into the cabin, passengers would typically hear the engine making noises, said Jake Fisher, senior director of auto testing at Consumer Reports. Corrosion on an older car, such as the one involved in the Marines’ deaths, can cause the hood to fill up with exhaust gases, which Fisher said can then get sucked into the cabin through an intake cavity between the hood and the windshield.

“You will absolutely hear a noise,” he said. “There would be a lot of warning, and that’s why a case like this is very rare.”

Although carbon monoxide has no odor or color, an exhaust leak would also release other chemicals with a noticeable smell, Fisher said.

Prolonged exposure to carbon monoxide reduces the ability of blood to carry oxygen to the body’s organs. It can cause throbbing headaches, disorientation and drowsiness, followed by unconsciousness, convulsions and eventually death.

It’s nearly impossible for carbon monoxide poisoning to occur in a vehicle without notice, Fisher said, unless the passengers are already asleep or impaired.

Officials haven’t released a toxicology report or explained the details leading up to the Marines’ deaths.

They could have been resting at the gas station with the air conditioning on and set it to recirculate cabin air, said Greg Brannon, director of automotive engineering for AAA. If exhaust fumes had seeped inside, air conditioning set to recirculate would not pull in any outside air to mix with the exhaust, causing the poisoning.

“Trying to take a nap in a running car is never a good idea, in my estimation,” Brannon said. “The recirculating air is the most efficient way to cool a vehicle. And also more dangerous for this very reason.”

If the air conditioning had not been set to recirculate, it could have pulled in fresh air and pushed out the contaminated air, he explained.

Three Marine lance corporals from Camp Lejeune died in the incident, including Tanner J. Kaltenberg, 19, of Madison, Wisconsin, Merax C. Dockery, 23, of Seminole, Oklahoma, and Ivan R. Garcia, 23, of Naples, Florida. Sheriff’s deputies found them on an early Sunday morning, about 30 miles (50 kilometers) southwest of their base, after the mother of one of the Marines reported her son missing.

Rust likely formed holes in the car’s exhaust and floor, letting fumes from the engine into the passenger compartment, Brannon said. Salt exposure from the ocean can cause rust, Fisher said, and older car parts can develop leaks over time. Garcia’s 2000 Lexus had traveled with him from Florida.

If the car had also spent some of its life in northern states where corrosive salt is used to clear the roads of snow and ice, holes from rust formation would be highly probable, Brannon said.

Sitting in an idling car for a long time is usually safe, Fisher said. But drivers should keep an eye out for warning signs and have them inspected annually. Vehicles are more prone to exhaust leaks after a crash and should be inspected before they are put back on the road.

“Engines emit a lot of very dangerous chemicals and gases,” Fisher said. “If your car is not running right and you hear it sounding funny, you really do need to get it checked out.”

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Associated Press auto writer Tom Krisher contributed reporting from Detroit.