AMARILLO, Texas — A poisonous gas believed to have been released when someone tried to wash away a pesticide that had been sprayed under a Texas home killed four children and left six other people hospitalized, officials said Monday.

Phosphine gas was likely released when water mixed with the pest control chemical, fire officials said. A specific cause of death had not been released for the four children Monday. 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, fire Capt. Larry Davis said. 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 found everyone sick.