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Police are seen outside the U.S. Holocaust Memorial Museum in Washington, D.C., on Wednesday. ((Gerald Herbert/Associated Press))

A security guard was shot to death after an elderly man said to have ties to white supremacist groups walked into the United States Holocaust Museum in Washington, D.C., on Wednesday and opened fire, officials say.

The man shot the guard with a rifle as soon as he entered the building at 12:52 p.m. ET, said Cathy Lanier, the chief of D.C.'s city police. The guard was taken to George Washington University Hospital, where he later died.

Two other security guards at the museum fired at the shooter, said Sgt. David Schlosser of the U.S. Park Police, which polices several National Park Service areas.

The shooter was also taken to hospital, where he is in critical condition, Washington Mayor Adrian Fenty said.

Fire department spokesman Alan Etter told CNN that a third victim suffered minor injuries, likely from flying glass.

Law enforcement officials said that the suspect in the shooting was 88-year-old James Wenneker von Brunn, a native of Maryland. Von Brunn is well known to police and civil rights organizations for his ties to hate groups and white supremacists.

Museum officials identified the dead guard as Stephen T. Johns, a six-year veteran of the facility.

David Unruh, who was visiting from Kansas, was among the tourists who hit the floor when they heard the gunfire then ran for the exits of the museum.

"We were scared to death. We got as low as we could get," Unruh said. "We didn't know exactly what had happened except someone was shooting guns and everyone was screaming and excited.

"You know you don't expect that, you're in a secure place, a place of reverence and respect. You don't expect gunshots."

Several loud pops

Mark Lippert was in the museum when he heard several loud pops and saw several schoolchildren running toward him.

"All of a sudden three kids ran into the area where we were and they had looks on their face like I've never seen before and it was just a horrified scared look, I knew something was absolutely wrong," he said.

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Law enforcement officers secure the perimeter of the U.S. Holocaust Memorial Museum in Washington on Wednesday after at least two people were shot at the museum. ((Haraz N. Ghanbari/Associated Press))

His first priority was then to find an emergency exit and get out, he said.

Another witness, who identified herself only as Maria, told CNN she saw the security guard being shot.

"He was face down. His back, I think, was shot, [and] blood was coming out," she said.

Linda Elston, who was visiting the museum, said she was on the lower level watching a film when she and others were told to evacuate.

"It was totally full of people," Elston said. "It took us a while to get out."

At the White House, just a few blocks away, press secretary Robert Gibbs said President Barack Obama was "obviously saddened by what has happened."

In Canada, officials with the Canadian Jewish Congress said they were shocked and saddened about the shooting.

"The Holocaust Memorial bears witness to the world about the atrocities and mass murders committed during the Shoah. To bring the tools of violence and to mount an attack in such a place dishonours the memory of those who suffered and perished during that dark period in human history," said congress president Mark Freiman.

Added CEO Bernie Farber: "The very idea that this type of violence would take place inside a Holocaust memorial is almost unfathomable in this day and age. Today's attack sustains the critical need for ongoing vigilance and rigorous security precautions."

Heavy security

The museum was closed after the shooting, and there were no reports of any further injuries, Etter said. Roads surrounding the museum were also closed.

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.

Schlosser wasn't able to say whether the shooter had passed through the metal detector.

The museum, located just off the National Mall near the Washington Monument, is a popular tourist attraction that draws about 1.7 million visitors each year.

The suspect, von Brunn, has described himself as a victim of a court system controlled by Jews and blacks.

He has a racist, anti-Semitic website and wrote a book titled Kill the Best Gentiles, alleging a Jewish "conspiracy to destroy the white gene pool," the Associated Press reported.

In 1983, he was convicted of attempting to kidnap members of the Federal Reserve Board and served more than six years in prison. He was arrested two years earlier outside the room where the board was meeting, carrying a revolver, knife and sawed-off shotgun. At the time, police said von Brunn wanted to take the members hostage because of high interest rates and the nation's economic difficulties.

Writings attributed to von Brunn on the internet say the Holocaust was a hoax and decry a Jewish conspiracy to "destroy the white gene pool."

With files from The Associated Press