"We heard three, four, five pops," said Sen. Bob Casey, D-Pa. Police ordered Casey and nearby tourists to crouch behind a car for protection, then hustled everyone into the Capitol.
Others witnessed the incident, too.
"There were multiple shots fired and the air was filled with gunpowder," said Berin Szoka, whose office at a technology think tank overlooks the shooting scene.
The shooting comes two weeks after a mentally disturbed employee terrorized the Navy Yard with a shotgun, leaving 13 people dead including the gunman.
Before the disruption, lawmakers had been trying to find common ground to end a government shutdown. The House had just finished approving legislation aimed at partly lifting the government shutdown by paying National Guard and Reserve members.
Capitol Police on the plaza around the Capitol said they were working without pay as the result of the shutdown. A spokesman for House Majority Leader Eric Cantor said a bill to pay them was under consideration.
Associated Press writers Bradley Klapper, Laurie Kellman, Adam Goldman, Mark Sherman, Philip Elliott, Jesse Holland, David Espo, Alan Fram, Brett Zongker, Donna Cassata and Henry C. Jackson in Washington, Michael Melia in Hartford, Conn., and John Christoffersen in Stamford, Conn., contributed to this report.