US Park Police Sergeant David Schlosser describes the shooting
An 88-year-old man reputed to be a violent racist has opened fire in front of crowds of people inside Washington DC's Holocaust museum, killing a guard.
The gunman, named by local officials as James von Brunn, was himself shot by police officers. He is said to be seriously ill in hospital.
American media reports say the man is a white supremacist who has served time in prison for violent offences.
US President Barack Obama said he was "shocked and saddened" by the incident.
"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 security guard who was killed was named as Stephen Tyrone Johns
Israel's embassy in Washington also condemned the attack.
The dead security guard was named as Stephen Tyrone Johns, who had worked at the museum for six years and "died in the line of duty", the museum said.
Joseph Persichini, assistant director in charge of the Washington FBI field office, said the shooting was being investigated as a possible hate crime or a case of domestic terrorism.
Soon after the shooting federal agents were dispatched to von Brunn's home in Annapolis, Maryland, to check his computer.
Metropolitan Police Chief Kathy Lanier said the gunman appeared to have acted alone.
She said he had been "engaged by security guards immediately after entering" the museum.
James von Brunn served time in prison for a 1981 gun incident
Earlier, police Sergeant David Schlosser said the gunman had walked into the building carrying "a long gun" and opened fire on a guard.
"My understanding is that two other security officers at the museum returned gunfire at the man that had entered the museum," he said.
A law enforcement official quoted by AP said von Brunn's vehicle had been found near the museum and was being tested for explosives.
Von Brunn has long been tied to extremist groups and served a prison sentence for taking a sawn-off shotgun, revolver and knife into Washington's Federal Reserve in 1981, when the board was meeting.
He has a racist, anti-Semitic website and has written a book about the "destruction of the White Race".
'Screaming and ducking'
The museum was crowded with schoolchildren and tourists at the time of the attack but all escaped injury.
Some 1.7 million people visit the museum each year
One eyewitness, 22-year-old Angela Andelson, told AFP news agency she heard a loud bang "like someone had dropped something".
She said she saw a man with "a long-looking kind of gun".
"I just ran in to one of the exhibits to try to take cover," she said.
"I heard the first one. When I turned and looked there were maybe two to four more shots that I heard.
"People were screaming and ducking down getting on the floor, getting under benches."
The museum normally has a heavy security presence with guards positioned both inside and outside.
All visitors are required to pass through metal detectors at the entrance and bags are screened.
Thirty million people have visited the museum, located on the National Mall, since it opened in 1993.
The museum has been threatened by anti-Semitic extremists before.
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