A Boston police officer stands on a closed-off Boylston Street April 16, 2013 in Boston, Massachusetts, in the aftermath of two explosions that struck near the finish line of the Boston Marathon April 15.A massive probe was underway Tuesday after two bombs struck the Boston Marathon, killing at least three and wounding more than 100. Monday’s blasts near the finishing line raised fears of a terrorist attack more than a decade after nearly 3,000 people were killed in suicide airliner strikes on New York, Washington and Pennsylvania on September 11, 2001. US President Barack Obama went on national television to warn against “jumping to conclusions” but a senior White House official, speaking on condition of anonymity, said such an attack was “clearly an act of terror.” AFP PHOTO Stan HONDA