U.S. investigators on Sunday were studying possible links between a pair of bombs detonated in New York City and New Jersey over the weekend, although no evidence had yet emerged tying the devices to known extremist groups.
The country was shaken by a trio of attacks over the weekend including a Saturday night bombing that injured 29 in Manhattan and a stabbing attack at a Minnesota shopping mall that wounded nine.
While officials described all three as deliberate, criminal acts and were investigating them as potential "acts of terrorism," they stopped short of characterizing the motivation behind any of them until more evidence is uncovered.
A deafening roar and powerful shock rocked Manhattan's popular Chelsea neighborhood late on Saturday after a pressure-cooker bomb packed with shrapnel exploded. A similar, unexploded device, was found a few blocks away later that night.
No international militant group immediately claimed responsibility for the New York blast or a pipe bomb that went off earlier along the route of a Saturday road race in suburban New Jersey. But New York Governor Andrew Cuomo said the act of blowing up a bomb in a crowded area of Manhattan "is obviously an act of terrorism."