Investigators say a toxic chemical used to exterminate rodents may have wafted into a Layton home, killing a 4-year-old girl.

The girl's mother initially called fire crews Friday night after a carbon monoxide detector sounded at their home, near 1500 North and 2400 West. Carbon monoxide readings in the air and in one of the residents' blood were slightly elevated, but not dangerously so, said Layton Fire Chief Kevin Ward. Questar Gas crews ventilated the house and investigators found no leaks in carbon monoxide sources.

"Vehicles, furnaces, all the typical things -- we couldn't find anything," Ward said.

On Saturday night, the mother brought the 4-year-old to the Wee Care pediatric clinic near the Davis Hospital because the child was struggling to breathe, Ward said. The girl was taken to the hospital, where she died.

The girl's mother told authorities that the entire family -- including the father, and three other children, ages 14, 3 and 15 months -- had complained of flu-like symptoms that night, Ward said. The family reported that an exterminator had placed chemical pellets in burrow holes in the lawn on Friday, and investigators suspect the toxic chemical infiltrated the home. That chemical, phosphine, was detected outside the home, Ward said.

A team of hazardous-chemicals investigators with the Utah National Guard is inspecting the home and likely will have more detailed information this week on the concentrations of phosphine in the home, Ward said. They also are investigating whether that gas may have triggered the alarm on the family's carbon monoxide detector.

No neighbors have reported any symptoms of exposure to a toxic chemical, Ward said.

Authorities would not release the 4-year-old's name on Sunday pending notifications of extended family.

"It's just a tragedy," he said. Ward did not have details on the frequency or hazards of using phosphine for pest extermination.