Firefighters found what they believe is an unexploded device after the blasts that killed two people at Monday's Boston Marathon, a government official said.
At least 28 people were also injured in the explosions, Boston police said.
The marathon itself, on its Facebook page, referred to the blasts as bombs. Authorities did not say immediately what caused the explosions.
Massachusetts General Hospital said it was treating 19 injured people; Tufts Medical Center reported that some injured were to be treated there as well.
The explosions sent smoke billowing into the air at Copley Square, turning a site of celebration into a mess of destruction.
Out of an abundance of caution, the Lenox Hotel was evacuated, the Boston Globe reported, as authorities looked into possible security concerns.
Officials in other cities, including New York and Washington, D.C., tightened security as a result.
The explosions occurred at about 2:45 p.m., more than two hours after the first of the race's nearly 27,000 runners had crossed the finish line.
The race was halted as was subway service into the area.
The FBI and troops from the Massachusetts National Guard were assisting police.
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