John Gerrish, Ellen Chung and their 1-year-old daughter Miju were found dead alongside their family dog in the Sierra National Forest on Aug. 17

Authorities have revealed the cause of death for the California family mysteriously found dead with their dog along a hiking trail in the Sierra National Forest in August.

In a press conference on Thursday, the Mariposa County Sheriff's Office confirmed that John Gerrish, Ellen Chung and their 1-year-old daughter Miju died of hyperthermia and probable dehydration. Their 8-year-old dog Oski also suffered a heat-related death, police said.

"For Mariposa County, this is rare," Sheriff Jeremy Briese told reporters. "This is the first hyperthermia cause of death that I've witnessed here in 20 years."

The family of three and their dog were discovered dead in the Devil's Gulch section of the Hites Cove Trail on Aug. 17, just one day after they were reported missing by their nanny.

Jon Gerrish, Muji, Ellen Chung
John Gerrish, Ellen Chung and daughter Muji
| Credit: Rosanna Heaslett

As authorities worked to determine their cause of death, a portion of a river located near where the family was found was closed in September due to toxic algae.

Toxic or harmful algae, also known as blue-green algae, grows in warm, slow-moving and nutrient-filled waters, according to the Centers for Disease Control and Prevention (CDC). When ingested or swam in, toxic algae can cause sickness or death in humans and animals.

During the press conference on Thursday, police said they do not believe the family ingested the water in the area. They also noted how there were no filtration systems with them, but one 85-ounce Camelbak water bottle and a bottle with formula were found near the site.

The water inside those bottles was tested but came back with no toxins and appeared to be clean or tap water, according to police.

Later during the conference, the Mariposa Public Information Officer issued a statement from the family that read: "The loss of a close relative is pain almost beyond words. When that loss is multiplied by four, and one of those losses is a baby, that pain is indescribable."

The family went on to explain that "the lack of knowledge and certainty as reasons for the death" led them to frequently question the circumstances of the death for "all of the days and all of the nights."

"Our hearts will never forget the beautiful lives of John, Ellen, Miju and of course, Oski," the statement finished. "They will remain with us wherever we are and whatever we do."

gerrish family
Ellen Chung and John Gerrish
| Credit: Ellen chung/ instagram

When the family was found dead over the summer, officials said Gerrish was discovered in a seated position with Miju and Oski near him, according to The Washington Post. Meanwhile, Chung was found a ways away from them up a hill, the outlet reported.

The family and their dog reportedly did not have any physical wounds or signs of trauma on their bodies, leading the Mariposa County Sheriff's Office to initially rule out exposure to chemicals and use of a gun or another deadly weapon as potential causes of death.

In September, officials announced that all public areas along the Merced River between the towns of Briceburg and Bagby would be closed until at least Sept. 17 "in response to the presence of toxic algal blooms." The area was located downstream from where the family was first found, per the Associated Press.

Later that month, authorities ruled out six more potential causes, including suicide, lightning strike, exposure to carbon monoxide, carbon dioxide, or cyanide and illegal drugs and alcohol.

John Gerrish and Ellen Chung
John Gerrish, Ellen Chung
| Credit: Courtesy Steven Jeffe

Prior to their deaths, Gerrish and Chung had moved about 160 miles from their home in San Francisco to Central California during their pandemic, family friend Steve Jeffe told The Fresno Bee.

The couple made the move after Gerrish, a Silicon Valley software engineer, began working from home, Jeffe said. They wished to raise Miju away from a major city and hoped to trade in the bustle of the San Francisco Bay Area for the calm of nature, he explained.

"We're all just devastated," Jeff told the outlet. "They were really beloved by the people. A super generous, sweet and loving couple that was devoted to their daughter."

Gerrish was originally from England, while Chung worked as a yoga instructor before becoming pregnant with their daughter, per the Merced Sun-Star.