Police will continue to comb a "complex and horrific" crime scene for clues as to how a mother and her two children died after an incident at a home in Perth.
A 19-year-old man remains in custody assisting police with their inquiries after being arrested at Ellenbrook.
No charges have yet been laid, police say.
Police would not confirm the ages of the victims, but said it was a mother and her two children.
A male and female were found dead inside the home and another female was found critically injured in the backyard but died on the way to hospital, police say.
Assistant Commissioner Paul Steel told reporters in Perth on Sunday, a senior homicide squad investigating officer had said it was "potentially the most horrific crime scene he has ever been called to".
"It is a very complex and I would say horrific crime scene. It will take us some time to process, and that could be days," Mr Steel said.
"It's a crime that I know will send shock waves throughout the community of Western Australia.
"This is a terrible tragedy for the family involved, for their extended family and for those first responders who were forced to attend what was a horrific crime scene."
Police were initially called about 1.30am on Sunday to a convenience store on The Broadway before they went to the home. Both are still being processed as crime scenes.