This paper presents a qualitative study of occupant behavior in response to the September 11, 2001 World Trade Center disaster. Through social science-based analyses of transcripts from 245 face-to-face interviews with survivors from both World Trade Center towers, collected by project High-rise Evacuation Evaluation Database, a conceptual model was developed to describe the pre-evacuation period in what became the largest full-scale building evacuation in history. The objectives of this study were to understand the types of actions performed before occupants began evacuation via stairs and elevators, and why those actions were taken to improve techniques used in evacuation modeling tools. On September 11, 2001, occupants consistently developed new social norms and lines of action based upon the meanings that occupants assigned to the situation, including perceptions of risk, familiarity with the building and others in the building, and responsibility for others. These meanings were dependent upon the receipt of environmental cues as well as on pre-existing norms, experiences, training, and social roles. Published 2013. This article is a US Government work and is in the public domain in the USA.