Local police reported at least 2 persons dead, with 22 others injured, in the two explosions that went off at the Boston marathon finish line Monday.
The number of casualties is not certain yet, as police are trying to rein in the chaos that erupted after the explosions.
Additional unexploded explosives are reported to have been found in the area, some of them supposedly inactivated by the police.
Although there is no official position regarding the cause of the explosions, some officials have already referred to "bombing."
US President Barack Obama has been duly notified of the tragedy.
Traffic has been temporarily halted at Boston's Logan International Airport.
Security measures have been upped in Boston, New York and other US cities.