The Rayburn building was sealed because of the reported sound of gunfire in its garage. Almost five hours later, US Capitol police said they had reopened it after finding no sign of criminal activity.
A police spokeswoman said it appeared the sounds were made not by gunfire, but by construction workers in the garage of the Rayburn building.
"The explanation is that there were some workers who were working in the area of the Rayburn garage, in the elevator area. And doing their routine duties, they made some sort of a noise that sounded like shots fired," said spokeswoman Kimberly Schneider.
"We have returned to normal business."
No arrests or injuries were reported. One staffer was taken to the hospital after suffering a panic attack.
Source of report
Later, a US legislator admitted to being the source of the erroneous report of gunfire. Republican House member Jim Saxton, speaking to Fox television, said he was in the elevator at the garage level of the Rayburn House Office building, when he heard what he thought was gunfire.
The Capitol building was locked
down after the gunfire scare
"I heard what I thought to be between six and ten shots," the Saxton 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."
He continued: "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
Sounds of gunfire
Police reported they were investigating sounds of gunfire in a garage level of the Rayburn building shortly after 10:30am (1430 GMT).
Many members of the US Congress had left Washington, getting an early start on a weeklong congressional recess. The House was not in session and the Senate continued to meet in the morning.
Tactical teams searched the four-storey building, including its three underground garage levels and a Capitol Police firing range two floors away. Rayburn houses 169 lawmakers in three-room suites and has two basements.
"We are going door to door, floor by floor. Every square inch of the Rayburn building is going to be cleared out today," Schneider said earlier.