(CNN) -- Four children died after someone at their home sprayed water on a previously applied pesticide, causing a reaction that produced toxic phosphine gas, officials in Amarillo, Texas, said.
At least five other people were hospitalized in Monday's incident.
Fire Capt. Larry Davis said a family member had used water in an attempt to wash away the pesticide -- aluminum phosphide -- which had been applied under the home.
The incident preliminarily has been ruled an accidental poisoning, Davis said.
There were 10 people inside the mobile home at the time, according to Davis. Crews arrived at the scene just after 5 a.m. after receiving a call that something was wrong and that people at the address were sick.
First responders treated for possible exposure
The children who died ranged in age from 7 to 17. One died at the scene. Three died after being taken to the hospital, CNN affiliate KVII reported.
First responders also went to the hospital for treatment for possible exposure to the gas. Several were held some time for observation, according to KVII.
Aluminum phosphide is listed in the Toxicity Category I by the Environmental Protection Agency -- the highest and most toxic category. Specifically, the EPA points to the "acute effects via the inhalation route."
The substance is used to kill insects and burrowing rodents, especially in grain stores. When mixed with water it produces toxic phosphine gas.