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...