With the introduction of fUML, an OMG standard defining the operational semantics of a subset of UML and the conforming virtual machine, UML models can be used not only for informal design sketching but also for completely building executable systems. Although this has been an important step for UML, the full potential of having executable UML models, such as enabling runtime analysis and adaptation, cannot be realized using the standardized virtual machine due to the lack of the adequate means for accessing important runtime information and controlling the execution of UML models. In this paper, we aim at establishing the necessary basis to overcome this limitation. Therefore, we introduce extensions of the standardized fUML virtual machine in terms of a dedicated trace model, an event model, and a command API. We provide an open-source implementation of the proposed extensions, as well as a model debugger for UML models based on this implementation to demonstrate the feasibility of the presented concepts.