A family of three that died along with their dog while on a hike in California’s Sierra National Forest in August died from “heat-related issues,” Mariposa County Sheriff Jeremy Briese said Thursday.
The deaths of John Gerrish, Ellen Chung, their 1-year-old daughter, Miju, and their dog puzzled authorities, who initially responded by closing recreation sites and trails in the area due to “unknown hazards.”
Briese noted that temperatures on the trail the family followed reached between 107 and 109 degrees the day of their hike, through at-times steep terrain. The family had one 85-ounce water container on them, which was empty at the time they were found. Portions of the trail also had very little shade, due to a fire in the area in 2018.
The dog’s cause of death technically remains undetermined based on the condition of the remains. But Briese speculated the 8-year-old Akita mix likely also suffered from hyperthermia and dehydration.
The family completed the majority of the 8-mile Hite Cove trail and were 1.6 miles from their car when they were found.
The sheriff said it’s the first hyperthermia death in the area he’s aware of.
“This is a very unusual, unique situation,” Kristie Mitchell, a spokesperson for the Mariposa County Sheriff’s Office, said shortly after their bodies were found on Aug. 17. “There were no signs of trauma, no obvious cause of death. There was no suicide note. They were out in the middle of a national forest on a day hike.”
The family had been hiking in a remote area near the Merced River where there is no cell reception when they went missing.
Testing revealed high levels of toxic algal blooms in the river downstream of where the family was found, prompting speculation it may have been responsible for their deaths.
While additional testing confirmed the presence of algae, Briese said toxicology reports show there was “no evidence” they ingested any of that water.
Other causes of death that were considered but ultimately ruled out included gunshots, extreme heat, lightning, carbon monoxide, carbon dioxide, cyanide exposure, illegal drugs, alcohol and suicide.
Investigators considered the possibility that toxic gasses could have leaked out of one of several abandoned mines nearby, but found no evidence that was the case.
“These algal blooms can produce toxins that can make people and pets extremely sick,” Bureau of Land Management Mother Lode Field Manager Elizabeth Meyer-Shields said in a statement announcing the temporary closure of public lands near the river in September. “The safety of visitors to our BLM-managed public lands is a top priority.”
This article originally appeared on HuffPost and has been updated.