John Gerrish and Ellen Chung’s home in Mariposa, Calif., was quiet on Aug. 16 when their 1-year-old daughter’s nanny arrived. The family — including their dog, Oski — was nowhere to be found. Their truck was missing, too. As the hours wore on without a word from Gerrish or Chung, who had set out for a hike the day before, a sense of panic began to set in.

The couple’s house sits near the head of Hites Cove Trail, and hours after the family was reported missing at about 11 p.m., the trailhead is where police started looking. A sheriff’s deputy found the couple’s truck parked near the trail’s entrance around 2 a.m. Nine hours later and 1.5 miles from the truck, in an area known as Devil’s Gulch, a search-and-rescue team found Gerrish, Chung, their daughter, Miju, and the dog.

They were all dead.

Gerrish was in a seated position with the baby and dog beside him, according to the Chronicle. Chung was a little farther up the hill.

Authorities still don’t know how they died. Autopsies yielded no conclusive results about the cause. A toxicology report, which could take several weeks, is pending. A necropsy is being performed on Oski, the dog, but the Mariposa County Sheriff’s Office said no other animals were found dead in the immediate area.

“It’s just so tragic and mysterious,” Steven Jeffe, a close family friend, told The Washington Post on Sunday evening.

Gerrish and Chung moved to Mariposa from San Francisco in March 2020, Jeffe said. The pandemic made the slow life look increasingly attractive for the young professionals. Gerrish worked at Snapchat, and Chung was working toward a master’s degree in family therapy, he said.

Snakebites have not been ruled out, although that cause is unlikely because there was no physical evidence of wounds. The lack of definitive answers has led investigators to imagine other, more unusual causes. Carbon monoxide poisoning from a mine or toxic algae in the nearby Merced River are among the leading theories.