The pesticide that killed an eight-month-old girl and sent four other children to hospital in critical condition was brought to Canada illegally from Pakistan to rid the family's Fort McMurray apartment of bedbugs, says the children's aunt.

The mother had placed phosphine tablets around last Tuesday, with the children beginning to show signs of illness Saturday afternoon, said Shazia Yarkhan.

The parents took the children to hospital Sunday.

Phosphine, a controlled substance in Canada normally used as a pesticide, was found in an unmarked container in the home, said Brad Grainger, deputy chief of regional emergency services.

It had been brought into the country illegally.

Phosphine was blamed in the mysterious deaths of two Quebec sisters travelling in Thailand as well as several other tourists in 2012.

Two boys, aged two and six years old, were taken to Stollery Children’s Hospital in Edmonton. Two others, aged four and seven, remain in a Fort McMurray hospital. All are in critical condition.

Yarkhan said Monday the children in Fort McMurray are doing OK, but the boys in Edmonton are getting worse.

The mother is also under observation in the Fort McMurray hospital.

A firefighter enters the apartment building where five children and a mother were poisoned by a pesticide used to fight bedbugs. The youngest child, an eight-month-old girl died. (CBC) The pesticide acts on the nervous system in a manner similar to sarin, an outlawed chemical weapon, said James Kehrer, a pharmacy professor at the University of Alberta.

"Children tend to be more vulnerable to agents like this," he said. "They have a higher metabolic rate, their nerves fire in a little different way, and they don't have necessarily as good a way to eliminate the chemical from the body once it gets in."

​Phosphine gas is also heavier than air so stays nearer the floor where children are likely to play, Grainger said.

It should be used only in well-ventilated areas, he said.

"It's not designed to sit in a house or a bedroom where we had found the chemical."

Cpl. George Cameron said the RCMP are treating the situation as a sudden death.

"Like any investigation that we have, we begin by ruling things out," he said. "And when I say that, we don’t believe at this time that this was an intentional incident, if you will, but we have to determine the exact cause of the death of this infant, why it happened, could it have been prevented."

The pesticide use appeared to have been isolated to the one apartment, he said.

The RCMP are not releasing the names of the children at the request of family members.

Neighbours on other floors not informed of death

The fire department received a call about a possible chemical threat in the Fraser Avenue apartment building on Sunday afternoon. The main floor of the building was evacuated around 3 p.m. local time.

All other building tenants were allowed to return home around 6:30 p.m. Sunday after air-quality testing determined it was safe.

Meanwhile, some residents who live elsewhere in the building are upset that they were not evacuated from the building.

Eamon Smithers lives with his family on the building’s fourth floor. He said he hasn’t heard anything from either the building’s manager or the RCMP about the death.

"We have a 16-month-old little girl here, and when it comes to toxic situations or chemical spills, I know that anyone can be in danger within the outlying area," he said.

"I know they did an air quality test yesterday and they cleared the building as safe — but the thing is we were not informed of it at all, which kind of baffles me and disturbs me."