NOVA looks back at the events of 9/11 and reviews the major investigations into the collapse of the World Trade Center.

In a follow-up to the Emmy Award-winning documentary "Why the Towers Fell," NOVA looks back at the events of 9/11 and reviews the major investigations into the collapse of the World Trade Center. What has been learned in order to improve the construction and security of our most important structures, present and future? How does the U.S. rate compared to other countries? What are the challenges facing architects, engineers and builders, and the obstacles preventing the adoption of new building codes? The film features incisive interviews with key investigators and engineers - including Leslie Robertson, who engineered the World Trade Center towers and takes viewers to the construction site of his current project in Shanghai, touted as the tallest structure in the world.