A toxic algae bloom is under investigation as one of the possible causes after an entire family of hikers were found dead on a remote trail in California.
Police are looking into whether water they drank may have been poisoned by the algae or whether other hazards may have contributed to the deaths.
John Gerrish, his wife, Ellen Chung, their one-year-old daughter, Miju, and their dog were all found dead on a hiking trail near Hite's Cove in the Sierra National Forest after a family friend reported them missing on Monday evening.
Police were baffled as there was no obvious explanation for the deaths.
Kristie Mitchell, a spokesperson for the sheriff's office said: "This is a very unusual, unique situation. There were no signs of trauma, no obvious cause of death. There was no suicide note."
The area where the bodies were found was closed off on Tuesday.
Poisonous gas leaks from a series of old gold mine shafts in the area was also a potential cause being considered, but the Fresno Bee newspaper reported on Thursday that Mariposa County Sheriff Jeremy Briese said he did not believe the mines were a factor. Dehydration is also being considered.
The bodies are now undergoing post-mortem examinations in Mariposa and toxicology studies will also take place.
Meanwhile, waterways in the area are being tested for toxic algae.
Their friend, Mariposa real estate agent Sidney Radanovich, said the couple were avid hikers.
He told the San Francisco Chronicle: "They were such a loving couple. He loved showing the baby all sorts of things and explaining them to her."
The remote area where the bodies were found had no mobile phone service, Ms Mitchell said.