A US lawmaker later admitted to being the source of the mistaken report.
Republican Representative Jim Saxton, speaking to Fox television, said he was in an elevator at the garage level of the Rayburn House Office building, when he heard what he thought was gunfire.
"I heard what I thought to be between six and ten shots," the Republican lawmaker said.
"It sounded exactly like gunfire to me. It was not of a backfire nature. It was the sharp crack as comes out of a weapon."
"I dove back into the elevator, rushed back to my office and asked my chief of staff to report what I had seen or heard to the Capitol Hill Police, which she did, and that started the chain of events that unfolded over the course of the day."
The report prompted police to lock down the Capitol dome building and Rayburn for more than five hours, while they searched in vain for a possible gunman, trapping lawmakers, visitors and office workers inside the congressional complex.
The House was not in session at the time, with the exception of a couple of congressional hearings, while debate in the Senate was suspended early as a precaution.
The scene was one of panic, as armed Capitol police officers surrounded the complex. No one was allowed to enter Rayburn, and at least four ambulances were deployed outside the building.
Meanwhile, hapless tourists who had been sightseeing at Congress -- one of Washington's top attractions -- were required to sit immobile on the floors of the congressional corridors while police conducted a laborious, room-by-room search of the building.
Police eventually reopened the Capitol building but kept the Rayburn building under seal, finally calling off the entire lockdown after five long hours, during which a large part of the US Congress was brought to a standstill.
Eventually Capitol police revealed that the misunderstanding was result of noise from nearby construction.
Capitol Police operations late on Friday finally gave an all-clear allowing members of Congress, staff workers and visitors to enter and exit the Rayburn building again.
The incident appeared to be the latest example of a capital city in the throes of post-September 11 jitters, and prone to fears of violent attack at every turn.
After the September 11, 2001 terror attacks, extensive security procedures were put in place, including metal detectors at all entrances of the complex, along with numerous video cameras. But nerves remain fragile with several security false alarms that have led to lockdowns and mass evacuations over the past few years.