Mystery as missing Brit dad, American wife, baby and dog are found dead on hiking trail as hazmat cops swoop on scene

A BRITISH dad, his American wife, their baby and their dog have been found dead in mysterious circumstances on a hiking trail in a US national park.

Jonathan Gerrish, Ellen Chung and their one-year-old daughter Miju were found on Tuesday evening in a remote area of the Sierra National Forest in California.

A family friend said that she last saw the family on Saturday and were reported missing by a relative on Monday.

Authorities located the family's car near the Sierra National forest gate the same day.

Their bodies were located two miles from their car in an area with no phone reception.

Police are treating the scene as a hazmat scene, Kristie Mitchell, a spokeswoman for Mariposa County Sheriff’s Office said.

The area where the bodies were found sits atop multiple disused gold mines with the local sheriff's office suggesting carbon monoxide effusion from the mines as a possible cause of death.

“It could be a carbon monoxide situation. That’s one of the reasons why we’re treating it as a hazmat situation,” she said.

Another explanation for the "bizarre situation" could be the family's possible exposure to toxic algae, police reported.

The recovery team will be "taking precautions" for any poisonous gases in the area, officials said although none have yet been found.

Sheriff Jeremy Briese said the scene "does not indicate a clear picture of what occurred or a clear cause of death".

The last contact they made was a photo they uploaded on Sunday at 6.45am.

Authorities said the family went missing at 11pm on Monday.

“This is never the outcome we want or the news we want to deliver, my heart breaks for their family," Sheriff Jeremy Briese said.