At a press conference this afternoon, before city officials were ready to confirm von Brunn as the suspect, Lanier said that though investigators were still in the preliminary stages of the investigation, they believed that the suspected gunman acted alone and that "no prior threats" had been made to the museum.
She added that an off-duty police officer was near the museum at the time of the shooting and immediately responded to the scene.
Joe Persichini, the assistant director in charge of the FBI's Washington Field Office, said those working on the joint investigation will be "working through the night to develop the facts" in the case.
Washington, D.C., Mayor Adrian Fenty praised the museum's security staff as heroes and said, "Our hearts and thoughts go out to the security guard and his family.
"In these days and times, you never know when someone is going to grab a gun and use it in an inappropriate way," he added.
Museum chief of staff Bill Parsons said the museum was full at the time of the shooting, with a "couple thousand" occupants at the time.
He also praised the security staff, saying, "They did exactly what they were supposed to do."
Eyewitnesses described the scene of terror and panic as the incident unfolded.
One told ABC News Radio that he heard five or six shots, and saw a man lying shot outside the building "in front of the front entrance." The witness said the victim appeared to be a security guard.
Another museum visitor, Dave Unrah of Wichita, Kan., said he was in the museum at the time of the shooting, and heard what he thought were firecrackers and a command for everyone to hit the floor.
Salt Lake City resident Katrina Rougelot and her daughter Miranda had been watching a film exhibit in the museum basement, and initially thought the gunshots were part of the film.
But she said a woman came in screaming that there had been a shooting. Panic ensued, and Rougelot and her daughter hid behind a statue. Security personnel ushered them out of the museum about 30 minutes later.
At a news conference this evening, Bloomfield said, "This is the kind of thing that we prepare for; we take security here very seriously."
D.C. Metro Police flooded the scene, cordoning it off with crime scene tape and closing streets around the museum.
Lanier said authorities swept all the areas around the museum as a matter of routine.
According to law enforcement sources, part of the plan in these situations is to flood the area with police and to make certain there are no accomplices.
As a precaution, the FBI's Joint Terrorism Task Force is also on the scene gathering information.
The museum, which is adjacent to the National Mall and approximately a mile from the White House, opened in 1993.
ABC News' Polson Kanneth and Andrew Miller contributed to this report.