At least two people were killed and more than 50 injured after two bomb explosions struck near the finish line of the Boston Marathon on Monday.
Security officials said a third explosion occurred at Boston's John F. Kennedy Library, located in another area of the city. No injuries were reported from that blast, and it is not certain whether it was related to the other two.
Boston police did not say whether the explosions were part of a terrorist attack. A police spokesman said bomb disposal teams were examining discarded bags and parcels at the scene of the initial blasts, but that no other unexploded devices have been found so far.
Television footage showed scenes of confusion, streets littered with debris and blood, paramedics carrying stretchers, and damage to nearby buildings.
Bloody spectators were carried to the medical tent that had been set up to care for fatigued runners.
The blasts occurred not long after the first of the race's nearly 27,000 runners had crossed the finish line. The race was halted after the explosions, as was metro service to the area.
The Federal Aviation Administration said Monday it had placed temporary flight restrictions in the airspace over the site of the initial twin explosions near the finish line of the marathon. The restrictions will not affect commercial air operations at Boston's Logan Airport.
A New York City Police Department spokesman said his city has increased security around landmarks in Manhattan, including near prominent hotels, in response to the blast.
Washington, D.C. and Los Angeles also are on a heightened state of alert.
Shortly after the explosions, the U.S. Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. But the White House is not on lockdown.
U.S. President Barack Obama called Boston Mayor Tom Menino and Massachusetts Governor Deval Patrick and told them his administration will provide whatever assistance is necessary in the investigation and response.
Boston is a major metropolis located in the northeastern U.S. The marathon is a significant event the city hosts every year. It attracts runners and spectators from all over the world.