Aggressive dog

The dog is out of control they cannot control it. Tonight a neighbour assisted with restraining it, to stop it going for another dog. The girl walking it cannot control it. It is very aggressive on the street, it tries to attack all the dogs. I feel sorry for the dog as it is a German shepherd and it is stuck inside they rarely exercise the dog. The dog barks most of the time. It just needs to get out and burn energy and exercise. I think they are keeping it in a cage. I walk my dog regularly and this dog is rarely out only late at night. I’m afraid it will Kill another dog given the chance