A woman died early Thursday morning in a single-vehicle highway crash near Lyman. A surviving 10-year-old boy kept his sisters warm in the truck for nearly four hours in near-freezing temperatures.

When highway patrol troopers arrived, Kymbrlee Smith, 30, of Green River, was already dead. Driver Samuel Smith, 31, of Green River, was flown to the University of Utah Medical Center, where he remained in critical but stable condition Monday, according to a Wyoming Highway Patrol news release. Three children in the vehicle were admitted to a nearby hospital and later released to family members.

A 2013 Toyota Tundra drifted off Interstate 80 around 2 a.m. Thursday, according to the news release. The truck hit a post near milepost 48, and when the driver tried to steer back onto the road, the vehicle hit a bridge guardrail. The pickup then rolled over the edge of the bridge and fell top-down onto the business road below.

The two adults, who both suffered head trauma, were unable to assist the three children. The boy helped keep his 3-year-old and 1-year-old sisters warm while they waited for help to arrive, according Sgt. Kyle McKay.

Around 6 a.m., roughly four hours after the crash, a person driving by spotted the Toyota and called 911.

The temperature ranged from 32 to 35 degrees in nearby Fort Bridger during those four hours, according to the National Weather Service.

The two younger children were both in child safety seats, and the other three occupants were properly wearing their seat belts, according to the news release.

The highway patrol Monday was investigating fatigue as the leading cause of the crash, according to the release.