At least two people were killed and 22 others injured after two apparent bomb explosions struck near the finish line of the Boston Marathon on Monday.
Television footage showed scenes of confusion, streets littered with debris and blood, and paramedics carrying stretchers.
Bloody spectators were carried to the medical tent that had been set up to care for fatigued runners. Police weaved their way through competitors as they ran back toward the course after the explosions.
Boston police officials confirmed they are looking into the explosions, but they had no further comment.
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 subway service to the area.
A New York City Police Department spokesman said the city has increased security around landmarks in Manhattan, including near prominent hotels, in response to the blast.
Washington D.C. also is on a heightened state of alert.
The White House says the administration is in contact with Massachusetts state and local authorities, and 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.