A Toronto police spokesman said Friday that a mother this week stabbed her husband and three-year-old to death before killing herself.

Detective-Sergeant Chris Buck, of the homicide squad, said that forensic investigation, interviews with family and neighbours and reviews of the 911 transcripts had implicated the mother in the deaths.

Brian Langer called 911 shortly after 7:30 a.m. Wednesday and said he had been stabbed. Police and emergency workers arrived a few minutes later to find a gory scene. Two-year-old Brigitte stood staggering on the front porch, blood pouring from a neck wound, wearing only a pink T-shirt and a diaper.

Zoe, 3, was found inside the house with no signs of life. Mr. Langer, 47, was alive at the scene, but died later. Andrea L'Abbé, 26, was found dead. The family dog was also injured. Only seven-month-old Margot was unharmed.

The post-mortem showed that all three deceased had died of puncture wounds, Det.-Sgt. Buck said. Zoe was killed by a wound to the neck, Mr. Langer by wounds to the torso and Ms. L'Abbé by a single stab wound to the neck.

"The deaths of both Zoe L'Abbé-Langer and Brian Langer, and the injury to Brigitte, were caused by the deceased person, Andrea L'Abbé," he said at a press conference Friday morning at Toronto police headquarters.

"I can tell you further that the wound to [Ms. L'Abbé] was self-inflicted."

He would not theorize on what might have led Ms. L'Abbé to stab Mr. Langer and two of their three daughters. He acknowledged speculation that she may have been suffering from postpartum depression but refused comment.

"This is a tragic case, there are no good answers for the families," he said. "We don't have all the answers as to the why, and we may never have them."

He told reporters that the families of the deceased had been informed Thursday night that Ms. L'Abbé was to blame.

He urged the media to respect the privacy of the grieving family and to allow them to give Brigitte and Margot a chance the chance to recover.

"Brigitte turns two years old tomorrow," he noted, "the families have to try to get past what happened this week and move on."

Peter Collins, a forensic psychiatrist with Ontario Provincial Police, refused to comment on the specific case but acknowledged that said some women become so severely depressed after having a baby that they become suicidal, and reluctant to leave their children behind to contend with what they see as a cold and cruel world.

"They're in a sense doing their kids a favour by killing them," Dr. Collins said. "Altruistic filicide is often associated with suicide."

In other cases, a woman may become delusional and paranoid, and may see her children or spouse or both as evil or threatening.

"One wonders whether there's been either some severe depression troubling her or whether she's in fact had some delusion that has frightened her," said Dr. Donna Stewart, a University of Toronto psychiatrist and expert in mental illnesses associated with giving birth.

With a report from Canadian Press