Four members of a family, two of them children, were found dead after an early-morning multiple shooting in southwestern Ontario on Monday.

It happened when gunfire erupted inside a modest residence in a quiet neighbourhood of London, Ont., about 110 km west of Toronto.

A nine-year-old girl called 911 and then ran for her life after her mother, brother and sister were shot in their home.

Neighbour Anne Alexander told CTV's Kitchener, Ont. affiliate CKCO that the sound of two gunshots woke her at about 2:00 a.m. ET

"I figured it was just fire crackers at first. And then I got up and I could hear sirens all over the place. There seemed like blocks of sirens, and then two policemen came," she said.

When the two officers approached the house, they were hit by gunfire, both in the upper body.

The officers, who were both wearing body armour, returned fire. And when they entered the residence, they found the shooter dead, along with three other victims -- the five-year-old brother, sister (believed to be 11 years old) and mother of the nine-year old girl, who managed to escape the horrifying scene alive.

Police say the man was not the father, but wouldn't reveal what his relationship was with the family. It's also unclear whether he shot himself, or died from a police bullet.

Police officers said they have never seen anything like the carnage they found inside the house.

"There is some emotional trauma that they're suffering right now, either because they have small children of their own or because of what they saw and what they had to do,'' said London Police Chief Murray Faulkner.

"Hopefully, our professionals can look after their needs."

Police have not yet identified any of the victims. Detectives and the Special Investigations Unit, which probes all incidents involving police and civilians that result in serious injury, sexual assault or death, have begun two separate investigations into the incident.

"The SIU will conduct their part of the investigation to determine what the extent and nature of police involvement was with the death of the male they found inside," said London police Constable Jeff Arbing . "And our major crime section is going to continue to work on the other deceased who were in the residence."

The two officers were released from hospital after being treated for injuries to the upper body and head.

Neighbours described the family who lived in the residence as quiet and said they kept to themselves. They said the husband worked as a trucker, and he was believed to have been driving in the U.S. when his family was murdered.

Police said there was also a fire at the home, located on a quiet residential street near the city's downtown core.

Two firefighters reportedly suffered smoke inhalation and were taken to hospital for treatment.

With reports from CTV's Scott Laurie and CKCO's Art Baumunk in London, Ont.