About collisions between planes and drones
 

Assuming both a drone operator and a plane pilot respected the aviation safety rules and there is a collision that ends with the crash of the plane, who will be declared responsible for the accident?

If somebody plays with a drone, he bought from a store, flying it below 400 feet just above his head in a place situated far from airports, and suddenly the drone is hit by a low flying crop duster that came from above a nearby forest, who will be responsible for the crash? The drone operator or the plane pilot?