A criminal investigation is under way into an accidental poisoning involving breathing a gas linked to a professional-grade pesticide that left four children dead and a woman in critical condition, police said.
Authorities are looking into why the family had the pesticide pellets, called Weevil-cide, which is only supposed to be sold to people with professional licences or certification and is marketed for use in rodent control in commercial transport of commodities and animal feed.
The father told first responders through a Spanish language interpreter that he had spread the pellets under the family's mobile home after obtaining the product from a friend, Amarillo Fire Captain Larry Davis said.
Mr Davis said the product is not available for sale to the general public. He added that the father does not have that certification, as far as he knew.
He did not know whether the friend who gave him the product had a certification.
Special crimes unit
Amarillo police spokesman Officer Jeb Hilton says the department's special crimes unit is investigating because of the child deaths.
Once completed, the investigation will be turned over to the district attorney to determine whether charges will be filed. Mr Hilton said other federal and state environmental regulation agencies may also investigate.
Fire officials said the children who died were three boys, aged seven, nine and 11, and a 17-year-old girl.
Officials have said all four children lived at the home in Amarillo, which is about 350 miles northwest of Dallas.
The children's mother, Martha Balderas (45), was in critical condition at University Medical Centre in Lubbock on Tuesday, according to a hospital spokesman.
Five other family members, including the father and four other children, were being treated at BSA Health System in Amarillo and were in stable condition, hospital and fire officials said.
Carbon monoxide suspected
Crews who responded to a 5am call to the home on Monday originally thought it was related to carbon monoxide poisoning.
Authorities later determined that phosphine gas was likely released when the father took a garden hose at some point on Sunday and tried to rinse away some of the pellets because family members had complained of the smell.
The water started a chemical reaction that released the phosphine gas. A visitor arrived early on Monday, found everyone sick and called 911.
Phosphine gas can cause respiratory failure and in severe cases can cause pulmonary oedema, which fills the lungs full of fluid.
About 10 first responders from the police, fire and medical response departments were also taken to the hospital as a precautionary measure, Mr Davis said.
Headache and nausea
Two were kept overnight for observation because of headache and nausea but were in good condition on Tuesday, he said.
Chip Orton, emergency management co-ordinator for the city of Amarillo and Potter and Randall counties, said his staff was working with a number of state and federal agencies to decontaminate the home.
The Texas Commission on Environmental Quality has hired a private contractor to help.
Mr Orton said emergency workers close to the home were wearing protective breathing equipment and hazmat suits as a precaution regarding the gas.
He said he does not know when the home will be safe for the family to return to.
AP