I don't believe he began firing from inside the farm house, but jmo. There appears to be some confusion in reporting. I think initial reports that the shooter began firing (with a silenced rifle) when the officer opened the unlocked door has lead many (including copy journalists) to just assume the shooter was inside the house.
I am now seeing more reports that seem to indicate that the shooter was dressed in camouflage and hiding in a cornfield from where he opened fire at the moment the officer opened the unlocked door.
There are media links supporting both accounts, but I think it makes more sense that he was in a close, nearby field. If so the officers at the door, even if standing aside the frame, would have still been in a direct line of fire
I've read that a drone was used by the officers to first scour the property (barn, farmhouse, at least some areas of field, etc.) before they approached the house. Im guessing he managed to go unseen due to the camouflage. A complete ambush.
It was in the rural area of North Codorus Township, when officials say Matthew James Ruth, 24, shot and killed three Northern York Regional Police Officers and injured two York
www.wfmz.com
Jmo