The cause of death has finally been determined in the case of the Mariposa County couple, their one-year-old daughter, and it was not toxic algae.

Mariposa County Sheriff Jeremy Briese gave a press conference Thursday afternoon to discuss the final determination of a two-month investigation into the deaths of married couple Jonathan Gerrish, 45, and Ellen Chung, 30, and their baby daughter Miju, as well as their dog Oski. And investigators have concluded that they all died from hyperthermia, or overheating, and likely dehydration, on what was an extremely hot day in Sierra National Forest, near the Merced River.

"Heat-related deaths are extremely difficult to investigate, and we want to thank you all for being patient with us," Briese said.

The case made national news, and the mystery of the family's deaths was cause for rampant speculation about the potential cause. The hiking trail where they were found on August 17 was treated as a hazmat situation, and both carbon monoxide from nearby mines and toxic algae from nearby water sources were talked about as possible causes of death. Also discussed was a random lightning strike that somehow hit all three people and the dog.

But like Berkeley marathon runner Philip Kreycik, who went missing in Pleasanton about a month earlier — and whose likely cause of death, because it has not been made official, was also hyperthermia — it was just one of this summer's ultra-hot days that was to blame. Due to likely delirium and wandering far off the trail he was running on, perhaps to seek shade under a tree, Kreycik's remains wouldn't be found for over three weeks.

Gerrish and Chung had relocated to Mariposa County from San Francisco during the pandemic after Gerrish got the opportunity to work remotely indefinitely, deciding they wanted to raise their daughter in a more bucolic setting. Gerrish, who was originally from England, worked as a software engineer for Snapchat.

Both were avid outdoor types, and the day they chose to go hiking in Devil's Gulch Valley started out fairly temperate. A witness saw them driving to the trailhead of Hite Cove Trail on the morning of Sunday, August 15, and around 8 a.m. when they set out, it was only 75 degrees out.

Investigators say that an app Gerrish used to map the trail did not indicate the steep elevation change on the trail, and the family hiked up to an area at 1,900 feet up, 2.2 miles up the trail, with little shade in what were likely 99-degree temperatures, as ABC30 reports. And temperatures may have gone well above 100 degrees, and the sheriff cited possible 107-degree temps in the area.

Hyperthermia and heat stroke can happen for people when temperatures hit 104 degrees.

As Bay Area News Group reports after the press conference, the family's route took them a total of 6 miles into a 7.5-mile loop, and they were found about 1.5 miles from their car, nearing the end of the loop. See the map of their hike below.

An 85-ounce water bladder, which was the only source of water the couple had with them, was found empty.

The sheriff noted in the press conference that this area lost much of its tree cover in the Ferguson Fire three year ago, and unless hikers were very familiar with it, they might not realize how hot it could get there.

The investigation into the deaths of Gerrish, Chung, and their daughter involved 30 local, state, and federal agencies.

A determination of the dog's cause of death was not given, but investigators believe it was also heat-related.