AMARILLO - Police officials in Texas say a criminal investigation is underway into an accidental poisoning involving pesticide that killed four children and left their mother in critical condition.

Amarillo Fire Capt. Larry Davis said Tuesday that the father told first responders he had spread a professional grade pest control pellet under the family's mobile home. Authorities later determined that phosphine gas was likely released when the father used a garden hose on Sunday to try to wash away the pesticide.

Davis says a professional certification or license is required to purchase the product, called Weevil-Cide. He says the father does not have that license. The father told first responders through an interpreter that he obtained the pesticide from a friend.

Police spokesman Officer Jeb Hilton says the department's special crimes unit is investigating because children were involved.