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Josh Gerrish, Ellen Chung and their one-year-old daughter Miju had been out with their pet dog on the Sierra National Forest trail, near an area known as Devil's Gulch.
They were reported missing at 11pm on Monday 16 August, Mariposa County Sheriff's Office said.
Search teams were able to locate the family's vehicle near the Sierra National Forest gate leading to Hites Cove in the Jerseydale area, before the family and their pet were found 'deceased'.
Mariposa County Sheriff's Office said in a statement: "The Mariposa County Sheriff's Office began a Search and Rescue operation after a family was reported missing Monday night August 16th at 11pm.
"Search teams located the family's vehicle near the Sierra National Forest gate leading to Hites Cove in the Jerseydale area. A short time later the family; John Gerrish, Ellen Chung, one-year-old daughter Miju and the family dog were located deceased near the Devil's Gulch area in the Southfork of the Merced River drainage."
Police said it is not initially obvious what happened, but confirmed the incident was being handled as a 'hazmat and coroner' investigation.
"Sheriff personnel with the assistance of the California Department of Justice are conducting the investigation and processing the area," Mariposa County Sheriff's Office said.
"Current scene information does not indicate a clear picture of what occurred or a clear cause of death, the scene is currently being handled as a hazmat and coroner investigation."
Sheriff Jeremy Briese added: "This is never the outcome we want or the news we want to deliver. My heart breaks for their family.
"Our Sheriff's Chaplains and staff are working with their family and will continue to support them during this heartbreaking time."
Kristie Mitchell, a spokesperson with the sheriff's office, said police believed the incident may be related to carbon monoxide.
She explained that there was no clear cause of death, meaning authorities are treating it as a 'hazmat situation'.
Mitchell said in a statement to CBS News: "It could be a carbon monoxide situation. That's one of the reasons why we're treating it as a hazmat situation".
Mitchell also told USA Today that there were no signs of trauma or wounds on the bodies, but that they weren't 'ruling anything out at this point'.
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