After American Airlines Flight 11 and United Airlines Flight 175 crashed into the twin towers of the World Trade Center and American Airlines into the Pentagon, Sliney gave the order to land every plane in the air over the US at the time (implementing the SCATANA plan), effectively shutting down US airspace.
After American Airlines Flight 11 and United Airlines Flight 175 crashed into the twin towers of the World Trade Center and American Airlines into the Pentagon, Sliney issued an order to land all planes in US airspace at that moment (implementing the SCATANA plan), effectively closing US airspace.