An eight-month-old baby is dead and four other children are in hospital after their mother used a fumigant to get rid of bed bugs in their Fort McMurray, Alta., apartment.

Shazia Yarkhan told CTV Edmonton that her sister was trying to kill bed bugs with a chemical brought from Pakistan, where the family had recently vacationed.

“She just wanted to kill bugs, and she just put some medicine over there in (the) apartment because she complained to the apartment caretaker and she didn't respond to anything,” Yarkhan said.

Police have not confirmed what type of substance was used, but sources told CTV News that it was phosphine, a dangerous agricultural pesticide that attacks the central nervous system. Phosphine is tightly regulated in Canada and a special licence is required to use it. The chemical is most often used by farmers on their crops.

Wood Buffalo RCMP Cpl. George Cameron said investigators are treating the Fort McMurray poisonings as accidental.

RCMP responded to a call for a reported chemical spill at 81 Fraser Ave. in Fort McMurray around 3 p.m. on Sunday. Although officers found that the spill was contained to one unit, they evacuated the entire building.

RCMP announced Monday that an eight-month-old baby died in hospital. Two of its siblings, aged two and six, were rushed by air ambulance to Stollery Children’s hospital in Edmonton. Two other siblings, a 4-year-old and a seven-year-old, were taken to Northern Lights Regional Health Centre hospital in Fort McMurray, police said.

All four children are in critical condition. Their mother is under observation in hospital in Fort McMurray, while their father is with the two children who were taken to Edmonton.

Police have not confirmed the gender of any of the children. No names have been released.

Yarkhan said her sister used the fumigant last Tuesday. On Saturday, her five children started to get sick.

The property manager also said that she spoke with the mother, who became alarmed when her children started vomiting.

Air quality tests conducted following the spill showed that there was no threat to other residents, who were allowed back into their units around 6:30 p.m. local time on Sunday.

With a report from CTV’s Alberta Bureau Chief Janet Dirks, CTV Edmonton and files from The Canadian Press