The area in the Sierra National Forest where the bodies were found on Tuesday had been treated as a hazmat site after concerns were raised about the deaths being linked to potentially toxic gases from old mines nearby.
Investigators are considering whether toxic algae blooms or other hazards may have contributed to the deaths of a California couple, their baby and the family dog on a remote hiking trail.
"This is a very unusual, unique situation," said Kristie Mitchell, a spokesperson for the sheriff s office. "There were no signs of trauma, no obvious cause of death. There was no suicide note."