WASHINGTON (AP) -- An elderly gunman opened fire with a rifle inside the crowded U.S. Holocaust Memorial Museum on Wednesday, gravely wounding a security guard before being shot. Authorities said they were investigating a white supremacist as the possible shooter.

The assailant was in critical condition, said Washington, D.C., Mayor Adrian Fenty.

D.C. Police Chief Kathy Lanier said the gunman appeared to have acted alone. He was "engaged by security guards immediately after entering the door" with a rifle, she said.

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.

But Lanier told reporters at a hastily convened news conference, "The second he stepped into the building he began firing.

Authorities said they were investigating for any possible connection with terrorism or hate crime.

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

Roads surrounding the museum were closed at least temporarily and blocked off with yellow tape. Several police cars and officers on horses surrounded the area.

One law enforcement official said James Von Brunn, a white supremacist, was under investigation in the shooting, and a second official said the elderly man's car was found near the museum and tested for explosives. They spoke on condition of anonymity, saying they were not authorized to discuss the investigation.

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