Regarding the police: I can't speak for those in your area, but my observation has been that sometimes they're reluctant to respond to complaints such as yours. My experience hasn't involved tenants as extreme as yours, but still--when it comes to charges like disturbing the peace--it's sometimes difficult to engage the police in a timely fashion. So you have to be very persistent. You say that the police are there constantly. That's a good start, and establishes some sort of history. Keep at that.
Regarding the landlord: Keep the pressure up there, too. Assuming his version of events is correct (and it may well be), you're still down low on his priority list. If they haven't paid rent in months, maybe the landlord doesn't have the money to evict the tenants. Or maybe he figures it's easier dealing with a periodic call from you than dealing with a really undesirable situation with tenants. You have to make your complaints with him a higher priority.
If there are other neighbors who are being affected by those rental homes, try to involve those neighbors as well. Get them to make some calls to the police and to the landlord.
If there's any sort of community or homeowners association, try to get it involved. Now, most of them don't like to get involved in such things, either. But if you become enough of a pest, you may get the community or homeowners association involved in your efforts.
There are two other possibilities to consider. The first is hiring a lawyer to assist you in your efforts. If you do, find one who's politically connected in your town. Someone like a former city councilman or former mayor. It may cost a bit more than a new law school grad, but it'll be well worth it.
The other possibility is making the landlord an offer to buy the properties. The landlord is probably highly motivated to sell, especially if the tenants aren't paying rent. So you make a very low offer. If there's still a mortgage on the properties (and it's low), you make the offer "subject to" the existing financing. Basically, he deeds the houses to you and you start making the mortgage payments. And there are plenty of services out there that specialize in evicting tenants. So you get the houses, evict the tenants, then put acceptable tenants in there instead. Or you sell the houses (for a profit) to someone who'd actually like to buy.
Hope that helps.
At that point - the local jurisdiction has the responsibility to force the landlord to comply and resolve the problem.
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