Investigators had been baffled for months by the deaths of a couple, their daughter, and their dog during a hike in the Sierra National Forest in August.

Law enforcement officials announced on Thursday, Oct. 21, that the Northern California family who died in August while hiking in the Sierra National forest died of hyperthermia and probable dehydration, Fox News reported.

The Mariposa County Sheriff’s Office believes Jonathan Gerrish, 45, his wife, Ellen Chung, 30, their one-year-old daughter, Miju, and their dog, Oski, an 8-year-old Aussie/akita cross died while hiking on the Hites Cove trail on Aug. 15. 

The temperature reached 109 degrees Fahrenheit (42.8 degrees Celsius) that day, and the majority of the eight-mile trail is devoid of shade or trees, Mariposa County Sheriff Jeremy Briese said.

On Aug. 17, the family was discovered dead near the Merced River on the Savage Lundy Trail in the Sierra National Forest. A friend reported them missing after Gerrish, a former Google software engineer, failed to show up for work.

“This is, again, an unfortunate and tragic event due to the weather,” Jeremy Briese said. “Heat-related deaths are extremely difficult to investigate.”

Their vehicle was found more than 1.5 miles from their vehicle, near a gate to the forest. The family’s 85-ounce water container was empty, and they did not have any extra water. There is also no mobile reception on the trail.

Foul play had been ruled out as authorities reported at the time that the remains showed no signs of wounds or trauma.

One theory investigated was the presence of harmful algae in the river. “Water testing along the Merced River near Hites Cove … show[s] a high concentration of algae bloom,” the California Department of Fish and Wildlife warned in July.

Toxic gases in the area were also suspected as possible causes of the deaths.

The family’s autopsies and a dog necropsy were also ruled inconclusive. Toxicologists, environmental specialists, and other experts have collaborated with the investigators.

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