MARIPOSA, Calif. (KSEE/KGPE) – A Northern California family found dead on a hiking trail in August died after they overheated and ran out of drinking water on a sunny August afternoon when temperatures reached 109 degrees Fahrenheit in the steep mountain terrain, authorities said Thursday.

According to Mariposa County Sheriff’s Department officials, the cause of death for Jonathan Gerrish, 45; Ellen Chung, 30; and Aurelia Miju Chung, 1; has been determined to be “hyperthermia and probably dehydration due to environmental exposure.”

Oski, the family dog, was also found dead. What killed the 8-year-old Australian shepherd and Akita mix wasn’t clear as of Thursday, but Sheriff Jeremy Briese said the dog was “possibly suffering from heat-related issues.”

“This is an unfortunate and tragic event due to the weather,” he said.

The case, which had baffled investigators, involved more than 30 law enforcement agencies who had painstakingly reviewed — and ruled out — causes such as murder, lightning strikes, poisoning, illegal drugs and suicide.

The family was found dead on Aug. 17 on the Savage Lundy Trail in Mariposa County.

Officials said the trail the family was hiking on was exposed to constant sunlight with very little shade, with temperatures between 107 and 109 F.

Officials found the family two days later after relatives had reported them missing. The family had hiked 6.4 miles (2.5 kilometers) with the baby in a backpack-type carrier. They were only 1.6 miles (2.5 kilometers) away from their car.

The family had an 85-ounce (2.5-liter) water container with them that was empty. A portion of the trail ran along the Merced River, where tests of the water showed it was contaminated with Anatoxin A, a lethal toxin produced by blue-green algae.

That prompted the Bureau of Land Management to close campgrounds and recreation areas along 28 miles (45 kilometers) of the river, between the towns of Briceburg and Bagby. But Briese said Thursday there was no evidence the family had drunk any of the river water.

Briese said the FBI is attempting to unlock one of the couple’s cellphones, saying the agency is “making good progress.”

“Our hope is that that cellphone will continue to give us more answers about that day,” he said.

Kristie Mitchell, the public information officer for the sheriff’s office, read a statement from unnamed family members during a news conference on Thursday.

“Some questions have been answered, and we will use this information as a way of helping us come to terms with the situation,” relatives said, according to the statement read by Mitchell. “Our hearts will never forget the beautiful lives of Jonathan, Ellen, Miju, and, of course, Oski. They will remain with us wherever we go.”

The Associated Press contributed to this report.