MARIPOSA COUNTY, Calif. — A family found dead in August on a hiking trail in a remote area of the Sierra National Forest died of hyperthermia and probable dehydration, officials with the Mariposa County Sheriff’s Office announced Thursday.

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Officials said Jonathan Gerrish, Ellen Chung and their 1-year-old daughter, Aurelia “Miju” Chung-Gerrish, died due to environmental exposure. The cause of death remains undetermined for the family’s dog, an 8-year-old Aussie/Akita mix named Oski, though officials believe the dog “was probably suffering from heat-related issues,” according to deputies.

Hyperthermia occurs when a person’s heat-regulating mechanisms fail in extremely hot environments, according to the National Institutes of Health.

In a statement released Thursday, Sheriff Jeremy Briese thanked the public for its patience as authorities from more than two dozen agencies investigated the deaths.

“Heat related deaths are extremely difficult to investigate,” he said.

Authorities said the Gerrish-Chung family embarked on a hike in the Hites Cove area west of Yosemite National Park on the morning of Aug. 15. In the ensuing hours, as the family hiked down the Hites Cove Trail and an adjacent U.S. Forest Service Trail, temperatures rose as high as 103 degrees, officials said.

The family eventually began hiking a steep incline section of the Savage Lundy Trail, where there has been little shade since 2018, when the Ferguson Fire scorched the area. Deputies said temperatures on the trail reached as high as 109 degrees that day. The Gerrish-Chung family had nearly completed an 8-mile loop hike when they died, the Fresno Bee reported.

Friends reported the family missing on Aug. 16, after a babysitter arrived at the family’s home and realized they weren’t home. A deputy later found the family’s vehicle at the Hites Cove trail head. The family was found dead on the Savage Lundy Trail at 9:30 a.m. Aug. 17 with no cause of death immediately apparent.

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Authorities found an empty 85-ounce CamelBack-style water bladder with the family along with a small amount of baby formula in a bottle, some snacks, a child carrier backpack and a cellphone, according to deputies and the Bee. The sheriff said there was no cellphone service in the area where they were found, the Bee reported.

Aside from the 85-ounce water bladder, no other water containers or water filtration systems were found with the family.

Officials from six different labs tested water in the area, including samples from the south fork of the nearby Merced River, where toxic algae were identified. However, deputies said they found no evidence that the Gerrish-Chung family ingested any water from the river or that the toxin produced by the algae, anatoxin-a, can cause human deaths. The toxin has been linked to animal deaths, authorities said.

Investigators also examined an old mine more than two miles from the area where the family was found.

“Our staff along with the California State Highway Patrol helicopter, H40, searched that area and found no evidence that the family located or accessed the mines,” deputies said in a news release.

In a statement released by the sheriff’s office, the Gerrish-Chung family thanked authorities for their “sympathy, empathy and total dedication trying to find some answers for us.”

“The loss of a close relative is a pain almost beyond words,” the statement said.

“When that loss is multiplied by four and one of that four is a baby of just one years old, then the pain is indescribable. When that pain is then further impacted by the lack of knowledge and certainty as to the reasons for their deaths, then the questions of why, of where, of when and of how, fill your mind all of the days and all of the nights.”

The family said that although some of their questions have been answered, “The question ‘why’ can never be answered and will remain with us.”

“Our hearts will never forget the beautiful lives of Jonathan, Ellen, Miju and of course Oski,” the statement said. “They will remain with us wherever we go and whatever we do. In the future, when we sit beneath the trees listening to the wind soughing through the branches, we will hear them, and we will remember.”

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