This is the final report on the National Institute of Standards and Technology (NIST) investigation of the collapse of the World Trade Center (WTC) towers, conducted under the National Construction Safety Team Act. This report describes how the aircraft impacts and subsequent fires led to the collapse of the towers after terrorists flew jet fuel laden commercial airliners into the buildings; whether the fatalities were low or high, including an evaluation of the building evacuation and emergency response procedures; what procedures and practices were used in the design, construction, operation, and maintenance of the towers; and areas in current building and fire codes, standards, and practices that warrant revision. Includes the main final report and 42 companion reports including detailed investigation program reports (8), and technical topic reports (34). The investigation was conducted by the National Construction Safety Team for the Federal Building and Fire Safety Investigation of the World Trade Center Disaster, Building and Fire Research Laboratory, NIST.