The scene that greeted first responders at a house in the riverside suburb of Mosman Park was in police’s own words “highly distressing.”

An entire family dead in their own home — a mother, a father and their two teenage boys.

A “devastating tragedy” is how Detective Acting Inspector Jesscia Securo put it, addressing the media just three hours after the awful discovery was made.

So what do we know about what happened?

Police in forensic suit are inspecting a crime scene in a house

Police are investigating the suspected murder-suicide in the house.  (ABC News)

Police swarm street

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Detective Acting Inspector Jessica Securo said police were called to the house at 8:15am on Friday after “a person known to the family” who had an appointment with them went to the property.

She said the person “became concerned and then subsequently contacted police”.

By 9am, dozens of emergency services personnel had swarmed the scene, including five ambulances and multiple police vehicles.

The normally quiet cul de sac and surrounding streets were quickly cordoned off as police tried to work out what had happened.

News travels fast in Perth, and the sheer volume of emergency services personnel in a suburban street soon attracted media attention.

Police in blue forensic suits are seen in a crime scene.

Police are seen spend all day investigating around the crime scene.  (ABC News: Keane Bourke)

But when St John Ambulance confirmed it had stood down three of the five ambulances it had sent to the scene, and had not sent anyone to hospital in the two that remained, it seemed apparent that the news would not be good.

A police email followed stating there were no safety concerns for the wider community.

Police are at the front yard of a house

Police are at the front yard of the house where four bodies were found in suburban Perth.  (ABC News)

At 11:30am, a sombre police media conference confirmed the worst — a family of four had been found dead in the house in what was being treated as a murder-suicide.

Detective Acting Inspector Securo said the family comprised a 50-year-old father, a 49-year-old mother and their two teenage boys aged 14 and 16.

The ABC has chosen not to publish the identities of the deceased while permission from the extended family is sought.

Four people dead after ‘devastating tragedy’ in Perth’s Mosman Park

Police linger around a suburban crime scene

WA Police are investigating a suspected murder-suicide in suburban Perth after finding two teenage boys and two adults dead in a home in Mosman Park.

The boys, Detective Acting Inspector Securo said, had “significant health challenges”.

The nature of their health issues has not been made public, but the family “had engaged with care services”, Detective Securo said.

While stressing the police investigation was “in its infancy”, she said police were in little doubt it was a murder-suicide.

Three pets at the house — two dogs and a cat — were also found dead.

Exactly how the family and their pets died has not been revealed.

Police have cordoned off a crime scene

Police have cordoned off the crime scene after four bodies were found in a house. (ABC News: Keane Bourke)

However, police said it was not violently.

“At this stage I’m not able to disclose how these persons are deceased,” the acting inspector said.

“However, I do reiterate that there doesn’t appear to be a violent incident that has occurred at the house.”

The timeline of events is still being worked out.

Police don’t know if the family died on Friday morning, Thursday night or earlier.

Police have been deployed to the crime scene

Homicide Squad Detectives are investigating this incident as a suspected murder-suicide.  (ABC News: Keane Bourke)

Detectives have been doorknocking the area, interviewing neighbours and reviewing CCTV to try to establish when the family were last seen.

The bodies were found in separate parts of the house, police said, and an undisclosed note was left at the scene.

The family were “not known to police at all” — in other words, they had no criminal records — and there had been no reports of family violence.