How do I handle a tip about my tenant dealing drugs?

[Tony Reynolds]

One of my duplex tenants wants to move after just 5 months claiming the other tenant is selling drugs and posing a threat of physical violence. The accused tenant has been there several years and has always been very dependable with no previous reason to suspect a problem.

What do I do? How do I investigate to discern if the claim has a factual basis?

[Jonathan Twombly]

@Tony Reynolds I remember asking a very experienced property manager what you do when you find drug dealing on the property.  She said that they always go to the tenant, tell them that they have received reports of the activity, and that the police will have to be called to deal with it, but you won't call the police if they just vacate.  She said they always left.