It could be something wrong like drug activity, new company air pollution and etc. 

Selling drugs? 

My childhood friends include murderers, prostitutes, drug dealers, and theives. 

You'll need to contact them to find a dealer in your area, as they don't deal direct with the buyer.  

Are you looking to buy and hold to generate cash flow, or wholesale (dealer) or flip?   

Do a drive during the day to see what they look like in daylight, and do a driveby at 8pm on a Friday night - because that's when the drug parties are in full flow.

I tell them, hey, I'm not a bank.We used to do perform->pay, and then we got into some very, very bad situations where the customer drug their heels paying us (The worst case was a guy paying out of his IRA... checks took 6 weeks to issue) and put me in a cash flow nightmare. 

As long as its not a D- area with where all the tenants are dealing drugs and what not. 

One of the houses has a history of drug users, not currently.

You may see signs of gang activity, drug dealing, prostitution, etc.