Building up proper situation awareness is one of the most difficult tasks in the beginning stages of large-scale accidents. As ambiguous information about the events becomes available, decision makers are often tempted to quickly choose a particular story to explain the events. Subsequent information that contradicts the initial story may easily be discarded and cognitive tunnel vision takes over. Our approach, as part of the COMBINED Systems project, is to prevent tunnel vision by providing critical thinking support. In a laboratory experiment with 60 participants, we tested this hypothesis by comparing the Critical Thinking tool with a 'no support' control condition and a 'minimal support' condition. Participants acted as crisis managers determining the likely cause of an accident based on different pieces of information. The results show a positive impact of the tool on both the decision making process and decision making effectiveness.