Authorities said that a healthy couple, their baby, and a family dog were all found dead on a remote hiking trail near Hite’s Cove in the Sierra National Forest in northern California.

Investigators are working to determine whether toxic algae blooms or other hazards may have contributed to the mysterious deaths of John Gerrish, his wife Ellen Chung, their one-year-old daughter Miju, and their dog, Fresno Bee reported on Friday, Aug. 20.

After a family friend reported them missing late Monday, Aug. 16, their bodies were discovered on Tuesday and airlifted out of Devil’s Gulch near the south fork of the Merced River on Wednesday afternoon.

“This is a very unusual, unique situation. There were no signs of trauma, no obvious cause of death. There was no suicide note,” said Kristie Mitchell, a spokesperson for Mariposa County Sheriff’s office.

Emergency responders initially treated the incident like a hazmat situation because of concerns about potentially toxic gases from old mines nearby, but the hazmat declaration was lifted on Wednesday.

“I don’t believe it’s connected to a mine,” Mariposa County Sheriff Jeremy Briese said. “We don’t know the cause. … We won’t rest until we figure it out.”

The bodies were transported to the coroner’s office in Mariposa County for autopsies and toxicology exams, the sheriff’s office spokesman said.

The State Water Resources Control Board said it was testing waterways in the area for any toxic algae blooms on Thursday.

According to Briese, the family was found more than three miles away from the hard rock gold mining operation site used in the mid-19th century, at the bottom of Hites Cove. His crews searched around where they were found to make sure they were not overlooking mines or other hazards.

Toxic algae blooms are being considered as a possible cause of death, but Briese said that “We’re unsure.” He added that the U.S. Forest Service had warned about the algae at the trailhead where the family was found.

“And from what I’ve gathered, these algae blooms are due to the recent drought,” Briese said, “but I don’t know too much detail about the toxicity of them.”

According to NBC News, the remote area where the couple, known to be avid hikers, were found had no cellphone service. The hiking trail ran through an area of forest known—particularly in springtime—to have spectacular wildflower displays.