Ontario's police watchdog is investigating after a Toronto officer shot a 52-year-old woman dead at a north Toronto home Friday morning.

The woman, identified as Sylvia Klibingaitis, was shot in the chest at around 9:30 a.m. outside a home at 51 Wedgewood Dr., near Yonge Street and Finch Avenue. She was rushed to Sunnybrook hospital with serious injuries and was pronounced dead on arrival.

Klibingaitis, who lived with her 83-year-old mother, was reportedly threatening an officer with a knife.

Police were responding to a call at the address.

Ontario's Special Investigations Unit, an arms-length agency that investigates cases in which an altercation with police results in death or serious injury, attended the scene. Three forensics investigators with the unit were there, and investigators interviewed Klibingaitis's mother.

It's not yet clear how many shots were fired. Some neighbours said they heard as many as three, but the SIU would only say in a news release that an officer's firearm was "discharged at least once."

Baffled residents returning home in the afternoon were asked to remain outside a line of police tape.