problems with landlord & tenants next door

I am a property owner with neighbors who are renting next door. They are not supposed to have pets, but they have 3 dogs and 2 cats which are constantly on our property. They are also having some very severe domestic incidences that make my wife and I fear for our safety. We have repeatedly asked the tenants and the landlord to do something about the animals, but to no avail.

Question: Who is legally responsible (landlord, tenant or both)and do we have any legal recourse in this situation?

I am confused. Why are they not supposed to have animals? Because the town says they can't? Or because the condo or homeowners association says that? Or because the same landlord you have has rules that say that?

If the lease says they can't but the landlord says it's okay, that's a waiver and you have little cause to complain.