Tenants accusing another tenant for dealing marijuana

[Sara W.]

My upstairs tenants recently wrote me an email telling me that she is suspicious of my downstairs tenant of dealing marijuana. She said her husband and her as well as our neighbor have seen strangers coming in and out of the downstairs tenant's unit with jars of marijuana. The neighbor has caught the downstairs tenant in video and has reported it to the police. My upstairs tenant has also filed a police report as well. I have yet to hear anything from the police or from the neighbor. My question is what's landlord's responsibility when it comes to this issue. Thank you!