The senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were hidden in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
Gov. Deval Patrick said Boston can expect "heightened vigilance" today. Copley Station, which is located close to the bombing sites, will remain closed. Certain exits off the Massachusetts Turnpike will also be closed. Several colleges in the area were also being closed today.
Last night, the Boston Bruins cancelled its game at the Boston Garden. The Boston Celtics game scheduled there tonight has also been cancelled.