If a landlord negligently allows a 3rd party to enter my rental and property is stolen, who do I sue?

When a tenant rents a property, part of what they are purchasing is an expectation of a reasonable degree of privacy. Even the landlord himself must give reasonable notice to enter the premises. In this case, my landlord granted access to my rental to a third party and much of my personal belongings were stolen. Do I sue the landlord or the thief?