07/25/2013 11:56 EDT | Updated 09/24/2013 05:12 EDT

Winnipeg police still searching for mother of two children found dead in home

WINNIPEG - Police say they believe the mother whose two children were found dead in their home is still alive and are urging her to contact police.

Officers were concentrating their search for Lisa Gibson, 32, on the Winnipeg family's riverside neighbourhood, Const. Eric Hofley said Thursday. Emergency personnel were on the Assiniboine River, which runs near the home, and the police helicopter was also being used to look for the woman.

"We're still treating this as we are searching for Ms. Gibson (and) that she's alive," Hofley said. "We have no indication at this time to do otherwise."

Police were called to the Gibson home in a leafy, quiet area on Wednesday morning and discovered a toddler girl and baby boy in critical condition. The two were rushed to hospital but died.

Officers have been searching for Gibson ever since.

Although there are reports the children died in the bathtub, Hofley said the cause of death was still being determined. Police have been careful not to call Gibson a suspect and say they are concerned for her well-being.

"Ms. Gibson was the last person to our knowledge with the children so, obviously, once she's located and spoken with, where the investigation goes from there remains to be seen," Hofley said.

He said there have been sightings of the strawberry-blond-haired woman. Some have turned out to be a cases of mistaken identity and others didn't come in quickly enough to be of much use.

Police have taped off a pathway leading down to the river from Gibson's neighbourhood and an impromptu memorial of flowers and stuffed animals has sprung up outside the family home.

Police have spoken to close family members, said Hofley, who added the investigation could take a while.

"We don't know the circumstances fully of what's transpired here. Our concern right now is for the safety and well-being of Ms. Gibson."

Gibson's state of mind is as much a mystery as her whereabouts. Her open Facebook page, which has since been restricted, was full of happy pictures of her two children: curly-haired Anna, 2, and Nicholas, almost three months. The page said Gibson works at Concordia Hospital in Winnipeg.

The page included a family photo that appeared to be taken at a hospital shortly after the birth of Nicholas, along with a proud birth announcement.

Neighbours said the family moved into the home about two years ago and largely kept to themselves, although they often had family visiting.

They said Gibson worked as a pharmacist but was home on maternity leave.

On Thursday, the Winnipeg Free Press reported that it had been told by a relative that Gibson was being treated for postpartum depression.