A further two bombs have been found close to the scene of two blasts in Boston.
A senior US intelligence official said two other explosive devices were found nearby the scene of initial explosions close to the finish line at the Boston Marathon.
A third explosion was heard about an hour after the first two after authorities warned spectators to expect a loud noise from a water cannon that police apparently were using to destroy one of the devices.
Shortly after the explosions, Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. Several Secret Service patrol cars also blocked off the entry points to the road.
The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.
The Federal Aviation Administration warned pilots that it had created a no-fly zone over the site of the explosions in Boston.
The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile (5.6-km) radius has been created over 811 Boylston Street. The zone is limited to flights under 3,000 feet (914 metres), which is lower than most airliners would fly except when taking off or landing.
The notice said the no-fly zone is effective immediately and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.
Homeland Security Secretary Janet Napolitano directed her agency to provide "whatever assistance" necessary.