Building up situation understanding is one of the most difficult tasks in the beginning stages of largescale accidents. As ambiguous information about the events becomes available, decision-makers are often tempted to quickly develop a particular story to explain the observed events. As the accident evolves, decision-makers can fail to revise their initial assessments despite contradicting information. Our approach is to reduce fixation errors and confirmation bias by providing critical thinking support. In a laboratory experiment with 60 participants, we compared the effect on decision making of a critical thinking tool, which requires the explication of evidence-conclusion relations in situation assessment, with two control conditions. Participants acted as crisis managers determining the likely cause of accidents. The results show a positive impact of the tool on both the decision-making process and decision making effectiveness. Participants did, however, take more time to arrive at a conclusion using the tool.