Washington police say an elderly white supremacist with a history of violent behaviour stepped inside the U.S. Holocaust Memorial Museum and opened fire Wednesday afternoon, killing a security guard before being wounded himself.

The guard died Wednesday afternoon in hospital and the gunman was in critical condition, said Washington Mayor Adrian Fenty.

According to police, the attacker was alone and was "engaged by security guards immediately" after entering the museum.

"The second he stepped into the building he began firing," Washington Police Chief Cathy Lanier said.

Officers are investigating James Wenneker von Brunn, an 88-year-old who runs a Web site devoted to white supremacy, in the shooting, a police source told The Associated Press.

Another source said a vehicle registered to von Brunn was found near the scene of the shooting and was being tested for explosives Wednesday afternoon. The weapon used during the shooting was a .22-calibre rifle, the source added.

Neither police sources gave their names because of the sensitivity of the investigation.

The security guard who died has been identified as Stephen T. Johns. Museum director Sara Bloomfield said Johns was a six-year employee at the museum who "died heroically in the line of duty."

A few blocks away at the White House, President Barack Obama said "This outrageous act reminds us that we must remain vigilant against anti-Semitism and prejudice in all its forms."

In addition to running a racist Web site, von Brunn wrote a book called "Kill the Best Gentiles" published in 1999.

Von Brunn was convicted for attempting to kidnap board members of the U.S. Federal Reserve in 1983, allegedly because of high interest rates. He served six years in prison for the crime.

In 1981, police found him outside a U.S. Federal Reserve meeting room armed with a sawed-off shotgun, handgun and a knife.

The museum, which has attracted 30 million visitors since opening in 1993, was crowded with school children when the gunfire erupted.

None of the children were physically harmed during the violence.

Student Ashley Camp was on a field trip with about 40 classmates when the shootout began and guards ordered the teens to flee the building.

"We had to sprint as fast as we could out the door," the 14-year-old student told The Associated Press.

"I thought it was the movie (part of a museum exhibit), but then everyone started screaming and running."

Shortly after the shootings, the Federal Bureau of Investigation sent officers to von Brunn's home in Anapolis, Md., to search his computer. The FBI is investigating the incident as a possible hate crime or case of terrorism.

Virginia Gerker, one of von Brunn's cousins, told the AP from St. Louis that she hadn't seen him in 50 years, and that he had been "disowned" by the family and was believed to be mentally ill.

According to the Southern Poverty Law Center, von Brunn's Web site was listed as a hate site in 2008.

Von Brunn was commissioned as an officer in the U.S. Navy and was discharged in 1956. He was also an artist who once painted a portrait of Rear Adm. John Crommelin, a U.S. naval officer who held anti-Semitic views, according to the Southern Poverty Law Center.

Two decades ago, von Brunn worked for the Noontide Press, which is an arm of the Holocaust denying Institute of Historical Review, the Centre said.

With files from The Associated Press