CREDIT: Canadian Press Andrea Labbe holds children Brigitte (left) and Zoe in a family photo. Brian Langer, 47, his wife, Andrea, 27, and daughter, Zoe, 3, died. Margot, seven months, and Brigitte, 2, survived.

But mothers have been known to kill their children in misguided acts of mercy, also known as ''altruistic filicide,'' that are usually associated with major depression or psychotic illness, experts say.

Dr. Peter Collins, a forensic psychiatrist with Ontario Provincial Police, refused to comment on the tragic madness that claimed three seemingly happy lives and shattered those of two tiny survivors - daughter Brigitte, whose second birthday is Saturday, and seven-month-old baby Margot.

But he 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,'' 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.

Police were saying little Friday about the motive behind Labbe's attack on her family. Nor would they comment on her mental condition or a report that she may have been suffering from post-partum psychosis.

''We know now the hows, we know the whos, (but) we don't have all the answers as to the whys,'' Det.-Sgt. Chris Buck told a news conference.

''We may never have all the answers to the whys.''

Police responded to an emergency call Wednesday morning from Labbe's 47-year-old businessman husband Brian Langer, who told dispatchers he had been stabbed.

They were greeted by a horrific scene.

Three-year-old Zoe's throat had been slashed and Langer had been stabbed repeatedly in the torso. Labbe had fatally stabbed herself in the neck. Langer and Zoe died later in hospital.

Daughter Brigitte was recovering from her injuries in hospital, while Margot survived unharmed.

Pictures of a paramedic hustling Margot's tiny form out of the house, swaddled in bloodstained sheets, have become one of the tragedy's most enduring images.

Police broke the news that Labbe was responsible for the attacks to almost 20 shocked and grieving relatives Thursday evening, and asked the media to respect their privacy.

''This is a tragic case,'' Buck said. ''There are no good answers for the families.''

Dozens of bouquets of flowers were left at the family's home, and many people who knew them wept openly in front of the house. Neighbours described the family as loving and happy, with no indications of trouble.

However, one newspaper report said Labbe had confided to a friend she was under stress and worried about her mental health.

Experts say as many as 70 per cent of women feel depressed after birth and another 13 per cent show more severe symptoms. One or two in a thousand become psychotic.

''If anybody's going to do an infanticide or homicide, it's those women who are the most likely,'' Stewart said.

The psychosis usually begins almost immediately after the woman gives birth, and is usually readily apparent to those who come in contact with her.

''The exception to that may be the woman who is very bright and who's able to somehow conceal it,'' Stewart said.

The root causes of post-partum depression and psychosis, which are similar to other mental illnesses, are not well understood. Scientists have ruled out hormonal or metabolic abnormalities, but speculate some women may be genetically disposed.

''There may be some women who are susceptible to these normal changes (of childbirth),'' Stewart said.

''If they have a genetic propensity to psychosis, this might trigger the first episode.''