From the Washington Post:
The Pentagon’s south parking lot was shut and Metro service was halted for about an hour yesterday after a man approached a police officer near a checkpoint and claimed that he had an explosive in his car.
Arlington County bomb technicians conducted a sweep of the vehicle and later determined that the man had a relatively harmless smoke bomb — which looked like a couple of sticks of dynamite or firecrackers — in the front seat. The device was removed shortly before 3 p.m., when officials reopened the parking lot and the roads and ramps surrounding the Pentagon. Metro rail and bus service I drive past the Pentagon every day. This explains show of force on Route 1 yesterday. When the Humvee with mounted machine guns are out, that’s always a sign that something is up.