US police closed a building on Capitol Hill in Washington DC for five hours after the sound of construction work was mistaken for gunfire.
Police had asked people in the building to lock their offices
The alert happened when shots were reported at garage level in the Rayburn House building, where members of the US House of Representatives have offices.
Workers were told to stay inside their locked offices during a police search.
Officers later said the noise had actually come from construction work, and issued the all-clear.
Giving news of the all-clear, Sgt Kimberly Schneider said: "It sounded like gunfire and we responded as such."
She told a press conference that workers in a lift shaft had made noises that "sounded like gunshots".
During the alert a woman was taken to hospital after suffering a panic attack, but was released a short time later.
The Rayburn building is on the other side of Independence Avenue from the Capitol complex. It is connected to the complex by underground passages.
The security operation was launched after a single telephone call to police at 1030 local time (1430 GMT) that reported "gunfire, shots fired, [or] a loud sound" inside the building.
Two women were later reported to have run out of the building and told police they saw a white man with a gun inside.
The House of Representatives is not in session, but a committee hearing was going on at the time of the incident.
Representative Peter Hoekstra, conducting the hearing, told Associated Press he interrupted a witness to request those attending to remain in the room and ordered the doors closed.
"It's a little unsettling to get a Blackberry message put in front of you that says there's gunfire in the building," he said.
Some witnesses pointed out that there was a firing range in the same building and on the same level as the garage.