A third person was slightly wounded but did not need further treatment, police said.
|Von Brunn spent time in prison over an
incident involving the Federal Reserve [AFP]
Adrian Fenty, the mayor of DC, said the attack appeared to be the work of one person but that officials were examining all possibilities.
As authorities investigate the shooting more information has emerged about Von Brunn, a Holocaust denier who runs a website detailing his white supremacist views and who once tried to kidnap members of the US Federal Reserve board.
Von Brunn, who was thwarted in that incident when a guard captured him outside a board meeting carrying a bag stuffed with weapons, was sentenced in 1983 to time in prison and released in 1989.
Joseph Persichini, the assistant director in charge of the Washington DC's field office for the Federal Bureau of Investigation (FBI) said authorities have dispatched officials to "a suspect's" home to check his computer, the Associated Press news agency reported.
He said they were investigating the incident as a possible hate crime or as "domestic terrorism".
"People were screaming and ducking down getting on the floor, getting under benches"
Angela Andelson, shooting eyewitness
Angela Andelson, visiting the museum from San Francisco, told AFP she heard up to five shots during the incident.
"I heard the first one. When I turned and looked there were maybe two to four more shots that I heard," said Andelson.
"People were screaming and ducking down getting on the floor, getting under benches."
Stephanie Geraghty, 28, told Reuters the armed man appeared to be a white male carrying a silver gun.
"I heard the first shot, it sounded like something had been dropped from the upper stories down," she said.
"The next two came really fast ... at that point, everyone took off. [There was] chaos, running."
"This outrageous act reminds us that we must remain vigilant against anti-Semitism and prejudice in all its forms"
Barack Obama, US president
The museum, within sight of the city's Washington monument, was closed following the shooting while nearby streets were cordoned off by police.
In a statement the museum expressed its "grief and shock" at the incident and said it would remain closed on Thursday with its flags at half-mast in mourning at the guard's death.
The building normally has a heavy security presence with guards positioned both inside and outside, while visitors are also required to pass through metal detectors at the entrance and bags are screened, the Associated Press news agency said.
But Kathy Lanier, DC's chief of police, said at a news conference the gunman opened fire as soon as he entered the building.
Barack Obama, the US president, said was "shocked and saddened" by the shooting and expressed condolences to the family of the slain guard.
"This outrageous act reminds us that we must remain vigilant against anti-Semitism and prejudice in all its forms," he said in a statement.
The Israeli embassy in Washington DC condemned the attack in a statement and said it was "closely following" the situation.
Robert Gibbs, the White House spokesman, said White House officials were receiving regular updates from the FBI, the Homeland Security Council and the White House
situation room on the incident.
The museum is a popular tourist attraction drawing about 1.7 million visitors each year.