The latest news coming out of the Pentagon. The official headquarters for the United States Department of Defense, the Pentagon was first constructed in 1941 and is based in Washington, D.C. The Pentagon is the largest office building in the United States, with over 26,000 people working in the building. On September 11, 2001, 184 people were killed (59 on the airplane and 125 in the Pentagon) when five al-Qaeda hijackers flew American Airlines Flight 77 into the western side of the building.