Jonathan Gerrish, his wife Ellen Chung and their one-year-old daughter Aurelia “Miju” Chung-Gerrish, had been hiking with their dog Oski on a sunny August afternoon when temperatures reached 43C.
Investigators were baffled when they found their bodies in steep mountain territory near the Merced River on the Sierra National Forest trail.
The case involved more than 30 law enforcement agencies who had painstakingly reviewed and ruled out murder, lightning strikes, poisoning, illegal drugs and suicide.
On Thursday, Mariposa County Sheriff Jeremy Briese confirmed the family died from hyperthermia.
The condition is caused when a person’s body temperature is dangerously high after exposure to hot, humid weather.
“This is an unfortunate and tragic event due to the weather,” Briese said.
It is unclear what killed their pet dog, an 8-year-old Australian shepherd and Akita mix, but evidence suggests it was also suffering from heat-related issues.
Briese described Gerrish, 45, as an experienced hiker who used an app on his phone to plot a route along the Hite Cove Trail.
The family and their dog began the eight-mile loop at about 8 am on August 15, hugging the south fork of the Merced River which is a popular spot to view wildflowers.
But many of the trees had been destroyed in a wildfire three years ago, leaving much of the trail with very little shade.
When the family started, it was around 23 degrees but the temperature quickly climbed during the day as the trail descended.
By the time they reached the steep uphill section of the hike known as the Savage Lundy Trail, Briese said it was 43C.
Officials found the family two days later after relatives had reported them missing.
The family had hiked 6.4 miles with the baby in a backpack-type carrier and they were only 1.6 miles away from their car.
The family had 2.5 litres of water with them that was empty.
A portion of the trail ran along the Merced River, where tests of the water showed it was contaminated with Anatoxin A, a lethal toxin produced by blue-green algae.
That prompted the Bureau of Land Management to close campgrounds and recreation areas along 28 miles of the river, between the towns of Briceburg and Bagby.
But Briese said there was no evidence the family had drunk any of the river water.
The FBI is attempting to unlock one of the couple’s cellphones, saying the agency is “making good progress,” he added.
“Our hope is that that cellphone will continue to give us more answers about that day.”
Kristie Mitchell, the public information officer for the sheriff’s office, read a statement from unnamed family members during a news conference on Thursday.
“Some questions have been answered, and we will use this information as a way of helping us come to terms with the situation,” the family said.
“Our hearts will never forget the beautiful lives of Jonathan, Ellen, Miju, and, of course, Oski. They will remain with us wherever we go.”