Police say letters found after an Ohio murder-suicide that killed three children indicate it was orchestrated by their grandmother and uncle, who were found dead with the youngsters in the family garage amid a disagreement over who should care for them.

Toledo police say the bodies of the five were found Monday in a car, and they may have died of carbon monoxide poisoning. A truck was running with hoses leading from the exhaust into the car.

Firefighters found the bodies after using a sledgehammer to force open a barricaded door to the garage. Two dogs and a cat also died.

A family friend says the children’s mother had asked their grandmother to care for them about three years ago, but had recently decided they should return home.