WASHINGTON, May 26: Police at Capitol Hill, the seat of both chambers of the US Congress, said on Friday they were investigating reports that shots had been fired in an office building connected to the main facility.

Capitol Police spokeswoman Sergeant Kim Schneider said the police dispatch centre received a phone call at 10.30am from an unknown caller who said shots had been fired in the Rayburn Office Building garage. The officer said there were no reports of injuries.

Authorities searched the building in response to the call. People in the congressional office buildings and the Capitol were informed of the investigation through office communicators and electronic devices.

Business in the Capitol itself was not affected but people were asked to return to their offices and not leave. The Senate continued with business. The House had adjourned for the Memorial Day holiday, although some committee hearings were taking place.

The Rayburn Building is considered the prime office space for members of the House of Representatives. The building is just south of the Capitol and connected to the main building by a system of tunnels.

Earlier during the day, authorities lifted a temporary lockdown order that followed reports of gunshots.

“Part of our investigation right now includes searching the building with tactical teams... and part of that search includes looking for anybody who might have a weapon,” Sgt. Schneider said.

Terror experts, appearing on various television channels, said it’s possible what was heard could have been an accidental discharge of bullets. “That kind of sound echoes,” said one expert.

Sergeant Schneider said she could not confirm reports that a woman was seen being wheeled out of the Rayburn building on a stretcher, but she added the incident could have been unrelated.

Security across the Capitol has been ramped up since the Sept 11, 2001, attacks. Evacuation and lockdown orders have been issued several times since the attacks in the face of perceived threats.