WASHINGTON COURT HOUSE, Ohio (AP) — Ohio investigators say an explosion of liquefied petroleum gas probably caused the house fire that killed a woman and her three grandsons the morning after Christmas.

The state fire marshal's office said Wednesday that the blaze in Washington Court House caused extensive damage, preventing investigators from determining the source of the gas leak or explosion. That means the cause of the Dec. 26 fire officially will be ruled as undetermined.

It killed 60-year-old Terry Harris and her grandsons: 14-year-old Kenyon, 11-year-old Broderick and 9-year-old Braylon.

The Fayette County sheriff says the boys had stayed with their grandmother because they didn't want her to be alone on Christmas night.

The sheriff says the way they were found near the front door indicated she was trying to help them escape.