Apparently he started shooting before reaching the metal detectors:
Washington, D.C. Police Chief Kathy Lanier said the gunman appeared to have acted alone. He was "engaged by security guards immediately after entering the door" with a rifle, she said, but "the second he stepped into the building he began firing." The suspect reportedly did not attempt to conceal his weapon and began shooting before reaching the museum's metal detectors.
http://www.cbsnews.com/stories/2009/...n5077874.shtml