"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.
U.S. Capitol Police on the plaza around the Capitol said they were working without pay as the result of the shutdown.
People standing outside the Supreme Court across the street from Congress were hurried into the court building by authorities.
The White House also was briefly locked down after the incident at Capitol Hill and the stretch of Pennsylvania Avenue in front of the compound was closed to pedestrians.
AP writers Adam Goldman, Mark Sherman, Philip Elliott, Jesse Holland, David Espo, Alan Fram, Eric Tucker, Brett Zongker and Donna Cassata contributed to this report.