THREE people are dead, believed to be a mother and two children, in an “horrific tragedy” in Ellenbrook.

Police were called to a convenience store on The Broadway, believed to be a 7-Eleven, about 1.30am this morning after a man entered and requested police attend.

As a result, they then went to a nearby residence and discovered two deceased people inside the home and a third victim, a female with critical injuries, in the rear garden.

An ambulance was called for the female who was taken to Royal Perth Hospital but she died soon after.

The deceased are believed to be a mother and two children, one male and one female, police said.

A 19-year-old man is in police custody and assisting police with their inquiries.

One senior homicide investigator has described the scene as one of the most horrific they had attended.

Metropolitan Region Assistant Commissioner Paul Steel said the police investigation was in its early stages and they were still trying to contact next of kin.

They were unable to release details about the victims, any injuries they may have received or a murder weapon.

He said detectives and forensics officers were undertaking a complex crime scene investigation at both the store and the home.

“This is a terrible tragedy and for the family involved for their extended family and for those first responders who were forced to attend a horrific crime scene,” he said.

The three dead and the 19-year-old who is in police custody all lived at the same address, however the exact relationship between those involved is still being investigated.

Assistant Commissioner Steel said it was a crime that would send shock waves throughout the community of Western Australia.

“I can assure the community that WA Police are applying all resources that we can to get to the bottom of what’s happened,” he said.

“There is a very complex and I would say horrific, crime scene in place and it will take us some time to process that crime scene and that could be days,” Assistant Commissioner Steel said.

“What I can tell you is that a senior Homicide Squad investigating officer said to me today that it is potentially the most horrific crime scene that he has ever been called to.”

Matthew Schreck, 22, who lives in a neighbouring street, said he was woken by screaming and yelling about 1am.

“I just heard a lot of screaming and yelling and a bit of fighting, then a few moments later the cops flew past down there and headed this way,” he said.

“It was real loud screaming and yelling, it sounded pretty heavy.

“I couldn’t hear what they were saying … it sounded like a man.”

Another neighbour said she had just gone to bed about the same time when she heard somebody running past her house.

She said she did not think the noise was unusual at the time.

Anyone with information about the incident should contact Crime Stoppers on 1800 333 000.