A pesticide commonly used to kill gophers, voles and other rodents is implicated in the death of a 4-year-old Utah girl and the critical illness of her 15-month-old sister. Rebecca Toone of Layton, Utah, died Saturday just hours after Fumitoxin pellets were dropped into burrow holes in the Toone family’s lawn to treat a vole infestation.

Rebecca Toone was the first to show symptoms of poisoning, and was rushed to the hospital on Saturday where she later died, according to The Salt Lake Tribune.

The entire family was then hospitalized with flu-like symptoms and discharged on Sunday, but 15-month-old Rachel Toone was readmitted Sunday afternoon in critical condition.

Fumitoxin aluminum phosphide pellets, which contain food as a lure, off-gas a chemical called phosphine when they come in contact with moisture. Short-term exposure to the gas causes symptoms such as dizziness, lethargy, nausea, vomiting and severe lung irritation.

Officials are trying to determine exactly how the gas reached the family’s home after an exterminator positioned about 1.5 pounds of the pellets along a sidewalk leading to the Toone’s front porch on Friday. Inspectors believe it may have collected in an open space under the stairs to the porch and then seeped into the house.

"We are still at a mystery as to how it actually got into the home, how it leached in. Whether there was a, some kind of pipe or cracks in the foundations, we don't know," Layton Fire Chief Kevin Ward told KSL.com.

The pellets were positioned about seven feet from the home and three feet from the garage, reports The Salt Lake Tribune. EPA standards require at least 15 feet between where the pesticide is applied and any place occupied by humans, domestic animals or livestock.

Raymond Wilson, president of Bugman Pest and Lawn Inc., insists the technician followed proper protocol, though he didn’t know where the pellets were placed in relation to the house.

The EPA proposed increasing the buffer zone around homes treated with phosphine gas from 15 feet to 100 feet in 1998, but the change was never adopted.