At least five other people were hospitalized in Monday's incident in Amarillo, Texas, they said.
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, CNN quoted Davis as saying.
There were 10 people inside the mobile home at the time, according to Davis.
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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 the channel.
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."
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