Unknown non-renters staying in my 4-family's basement. What now?

[Steve Sherman]

Hey BP! 

I recently purchased a 4-family in an okay area of St. Louis and with the building I've inherited 4 tenants. A month into ownership, a few neighbors started complaining about the neighbor from one of the units, that he makes loud noises at night, might be doing drugs and that most of the time, someone potentially known to him, stays in one of 2 basements of the building at night. I've sent him a lease violation warning letter, letting him know, to keep it quieter at night and letting him know that the basements are off limits. I've also posted a note on each of the 2 basement doors, that basements are not living quarters and you can't hang out and sleep in them at night. In addition, I'm hearing from one of the tenants, that some people might be selling drugs somewhere close to the building and then staying in my basement somehow. Please, advice on the M&Ps on this. I'm thinking the obvious thing is just to call the cops, any time a tenant tells me that there are non-renters in the basement, but maybe there is a better way. Please, advice!