Fire Capt. Larry Davis said a family member had used water in an attempt to wash away the pesticide - aluminum phosphide - which had been applied under the home.
Davis said first responders initially checked for, but did not find carbon monoxide in the home. Poisonous phosphine gas was released as a result.
A visitor to the home called 911 after arriving early Monday and discovering everyone was ill.
Police said they still need to verify that phosphine gas exposure caused the four children's deaths, and autopsies are scheduled for Tuesday, but they say the story is a cautionary tale for people who search out the strongest chemical available but aren't properly trained to use them.
Authorities say no charges are expected to be filed at this time.
Four other children of the household and their father are recovering relatively well while the mother is in serious condition and being treated at a Lubbock hospital.
Phosphine gas can cause respiratory failure and in severe cases can cause a pulmonary edema, which fills the lungs full of fluid.
In addition to the people inside of the home, two members of the police department, seven fire fighters and one person from American Medical Response were taken to the hospital to get checked out.
First responders also went to the hospital for treatment for possible exposure to the gas.
Medical experts who specialize in poison control say accidental poisonings from the active ingredient in the pest control substance that led to the death of four Amarillo children are not uncommon, but deaths from aluminum phosphide are rare. "We see mostly the overdoses".
Chip Orton, emergency management coordinator for the city of Amarillo and Potter and Randall counties, says his staff is working with a number of state and federal agencies to decontaminate the home. He did not know when it would be safe for the family to return. Six other people were hospitalized and were "not out of the woods yet", a rescue official tells the Associated Press.