U.S. Park Police Sgt. David Schlosser said the gunman walked into the museum with what he described as a "long gun." The gunman and a security guard were shot. Both were taken to the hospital, but the extent of their injuries wasn't immediately known.
U.S. Park Police initially gave slightly different information, saying three people had been shot. Fire department spokesman Alan Etter told CNN a third person was hurt after being cut by broken glass. Several witnesses said they saw the security guard on the floor and bleeding.
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 said park police SWAT teams were doing a secondary sweep of the building, but they didn't believe there was another gunman.
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 have been closed and blocked off with yellow tape. Several police cars and officers on horses surround the area.
Mark Lippert of Lasalle, Ill., said he was at the museum when he heard several loud pops and saw several schoolchildren running toward him, three with horrified looks on their faces.
He said when he saw the kid's faces, he knew someone had been shot.
Sandy Perkins of Massachusetts said her daughter, Abigail, called her shortly after the shooting. The teen was on a school trip to the museum and told her mother students heard several shots before they were told to leave the building.
Abigail said some of her friends from Holton Richmond Middle School in Danvers, Mass., were very shaken, but all were otherwise fine, Sandy Perkins said.
The teens did not see where the shots were coming from.
Linda Elston, who is visiting the museum from Nevada City, Calif., said she was on the lower level of the museum 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."
She said she didn't hear any shots and didn't immediately know why there was an evacuation. The experience left her feeling "a little anxious," she said.