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WASHINGTON (Reuters) - A mammoth U.S. House of Representatives office building reopened on Friday after a floor-by-floor search by police indicated sounds thought to be gunshots were probably noises made by construction workers, the U.S. Capitol Police reported.

The Rayburn building was sealed because of the reported sound of gunfire in its garage. Almost five hours later, U.S. Capitol police said they had reopened it after finding no sign of criminal activity.

A police spokeswoman told reporters 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.

Schneider added, "We've returned to normal business."

No arrests or injuries were reported. One staffer was taken to the hospital after suffering a panic attack.

Police reported they were investigating sounds of gunfire in a garage level of the Rayburn building shortly after 10:30 a.m. (1430 GMT).

Many members of the U.S. 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-story 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.