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AMARILLO (AP) — Officials said Monday they believe a poisonous gas released when someone tried to wash away a pesticide that was sprayed under a Texas home killed four children and left six other people hospitalized.

Phosphine gas was likely released when water mixed with the pest control chemical, Amarillo fire officials said. A specific cause of death was not released for the four children Monday afternoon. The other six people who were in the home are “not out of the woods yet,” fire officials said.

Crews who responded to a 5 a.m. call to the home originally thought it was related to carbon monoxide poisoning, Amarillo fire Capt. Larry Davis said in a statement. One child died at the scene and three others died at a hospital.

Officials didn’t release any identifying information, including the children’s ages or whether they were related. Davis said all four people who died were residents of the home.

Davis told the Amarillo Globe-News he wasn’t sure how long the residents had been exposed to the phosphine gas before a visitor arrived Monday and found everyone sick and called 911. Phosphine gas can cause respiratory failure and in severe cases can cause a pulmonary edema, which fills the lungs full of fluid, he said.

Davis said about 10 first responders from the police, fire and medical response departments were also taken to the hospital as a precautionary measure, but none had shown symptoms of illness.