Toxic algae that can cause 'extreme sickness' has been found in a California river downstream from where a family were mysteriously discovered dead.
A 28 mile stretch of the Merced River, between the towns of Briceburg and Bagby, was closed Friday after the Bureau of Land Management (BLM) received the results of water sample tests.
That means hiking trails and campgrounds along the stretch are no longer open to the public, three weeks after British Snap engineer Jonathan Gerrish, his wife Ellen Chung, their one year-old daughter Muji and dog were found dead along the trail.
Mystery continues to surround what caused the family's deaths, although investigators have so-far confirmed that they weren't killed by gases from a nearby mine, or with weapons such as a gun.
Friday's closure has further raised suspicions that the family have been fatally-poisoned from drinking water, although officials are still waiting to receive toxicology reports.
BLM spokesperson Elizabeth Meyer-Shields said of the algae discovery: 'These algal blooms can produce toxins that can make people and pets extremely sick.
'We will continue to monitor for the algae’s presence and look forward to when the public can safely recreate in the Merced River.'
She spoke three days after federal officials closed trails and campgrounds in California's Sierra National Forest, citing the presence of 'unknown hazards', near the area where the family died.
'As a precaution and to protect the public from unknown hazards in the area, The SNF decided to close several recreation sites, roads, and trails along the Merced River and its South Fork, until deemed safe for public use,' the Sierra National Forest (SNF) announced in a press release on Saturday.
Officials revealed that the closures of more than a dozen picnic areas, campgrounds and forest trails, effective from August 29 through September 29, are due to 'unknown hazards found in and around the Savage Lundy Trail.'
Jonathan Gerrish, wife Ellen Chung, their baby daughter, Muji, and their dog were found dead in August, with toxic algae since discovered in a river close to where they'd been hiking
The Sierra National Forest in California closed more than a dozen trails, campgrounds and picnic areas, citing 'unknown hazards'
Forest officials closed recreational areas effective August 29 through September 26
Gerrish, his wife and baby were found dead in the vicinity of this remote canyon near the Californian town of Mariposa
On August 17, search and rescue teams discovered the bodies of Jonathan Gerrish, 45, his wife, Ellen Chung, 30, their one-year-old daughter, Muji, and the family dog, Oksi, on the Savage Lundy Trail.
The notice announcing the closures stresses that signs warning the public of 'potentially harmful algae blooms' have been posted in the area sine mid-July.
On Monday, the US Forest Service closed all forests in California through September 17 because of raging wildfires, including the Dixie, the second-largest wildfire in state history.
Two weeks after the grim discoveries of the family's bodies in the remote part of Mariposa County, investigators still do not know how they died.
According to the latest update from the Mariposa County Sheriff's Office, based on autopsies and a necropsy performed on the parents, child and their dog, officials have ruled out as causes of death gunshot wounds or injuries inflicted by any other type of weapon, as well as chemical hazards, such as gasses from an old mine along the Savage Lundy Trial.
The bodies of the family of three and their pet were found on the Savage Lundy Trail, 1.5 miles from their parked truck
There have been signs posted in the area since mid-July warning the public of toxic algae blooms
Authorities are still awaiting toxicology reports and cell phone data before making any
