Lawrence Livermore National Laboratory
Despite claims by the U.S. government that the events of 9/11 were unexpected, from 1998 the Secret Service was "crashing planes into the White House ... on a simulation program provided by the military" during training exercises, according to a retired Secret Service agent who had been involved with running those simulations. When this individual and his Secret Service colleagues learned that a plane had crashed into the World Trade Center on the morning of September 11, 2001, one of those colleagues pointed to him and commented, "You know all about that." 
The existence of these plane crash simulations was revealed by Paul Nenninger, who worked for the Secret Service for 26 years, in a chapter he wrote for a book published in 2005. In 1997, Nenninger had been assigned to the Secret Service's James J. Rowley Training Center, just outside Washington, DC, in Beltsville, Maryland, where he served as program manager in charge of the Security and Incident Modeling Lab (SIMLAB).