DALLAS — A criminal investigation is underway into an apparent accidental poisoning involving a professional-grade pesticide that left four children dead and an Amarillo woman in critical condition, police said Tuesday.

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 licenses or certification in their use.

The product's manual, available online, outlines stringent guidelines for using the product including having two trained individuals apply it.