Published: Apr 29, 2010 9:02 PM EDT

LAYTON, Utah  - Prosecutors in a Utah city will file negligent homicide charges against an exterminator who applied rodent poison that was later linked to the death of two young girls.

Assistant City Attorney Steve Garside says he expects to file charges Thursday against Cole Nocks, who worked for Bugman Pest and Lawn in Bountiful.

Four-year-old Rebecca Toone (pictured) and her 15-month-old sister, Rachel, (pictured) died in February after their yard in Layton was treated with poison-laced pellets used to kill rodents.

Investigators believe toxic phosphine gas from the pellets seeped into the home and sickened the girls. Garside says an investigation found Nocks applied too much pesticide and that it was too close to the home. The Utah Department of Agriculture is also seeking to revoke Nocks' license and fine him $27,000.