“Shelter in place,” Capitol police officers responding to the crisis barked out.
The House, which had been in the midst of heated debates over how to end the government shutdown, immediately went into recess.
Members of Congress and their staffers scurried to their offices, closed the curtains, and locked themselves inside.
Meanwhile, a police helicopter carrying heavily armed officers landed on the Mall directly in front of the building who raced over to help.
There were unconfirmed reports that a suspect was in custody or has been shot.
The shots rang out near Constitution Ave., on the north side of the Capitol.
“I heard three distinct booms,” NBC correspondent Luke Russert reported.
From inside the Senate Press Gallery, which has windows facing in that direction, frightened tourists could be seen running near the Supreme Court on 1st Street.
UPDATE: The shooting unfolded after police chased a black car up Constitution Avenue toward the Capitol, said tourist Edmund Ofori-Attah, who walked toward the scene as the car stopped.
"Then I heard the gunfire" and hit the ground, he said.
Sen. Bob Casey, D-Pa., told reporters he was walking from the Capitol to the Senate Russell Office Building across the street when he noticed several police officers driving fast up Constitution Avenue on motorcycles.
"Within seconds of that," Casey said, "we heard three, four, five pops," which he assumed were gunshots. He said police ordered Casey and nearby tourists to crouch behind a car for protection.
In about two minutes, he said, the officers moved everyone into the Capitol.
FBI agents rushed to the scene and Senate Sergeant at Arms Terrance Gainer said: "There are reports of injuries."
After nearly an hour, police ended the lockdown.
The shooting comes two weeks after a mentally disturbed employee terrorized the Navy Yard with a shotgun, leaving 13 people dead including the gunman.
As a warning was sounded, the House abruptly went into recess and lawmakers left the chamber floor. The Senate also suspended business.
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.
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. Secret Service said the procedures were precautionary
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