The Pentagon Memorial, formally the National 9/11 Pentagon Memorial, located just southwest of The Pentagon in Arlington County, Virginia, across the Potomac River from Washington, D.C., is a permanent outdoor memorial to the 184 people who died as victims in the building and on American Airlines Flight 77 during the September 11 attacks. Designed by Julie Beckman and Keith Kaseman of the architectural firm of Kaseman Beckman Advanced Strategies with engineers Buro Happold, the memorial opened on September 11, 2008, seven years after the attack.