In an appearance in the White House briefing room, Obama urged Americans to be vigilant and to watch for suspicious activity a day after two explosions rocked the finish line of the Boston Marathon, killing at least three people and wounding scores more. Obama, briefed by FBI Director Robert Mueller, Homeland Security Secretary Janet Napolitano and other national security aides, said there is still much to be investigated in what was the worst attack on US soil since Sept. 11, 2001.There is no sense of a motive and no indication as to who planted the bombs and detonated them, he said, condemning it as a "heinous and cowardly act.""Any time bombs are used to target innocent civilians, it is an act of terror," Obama said. "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization - foreign or domestic - or was the act of a malevolent individual."The president ordered the US flag atop the White House lowered to half staff in memory of the victims.He said the investigation into the bombings is just beginning."It will take time to follow every lead and determine what happened. But we will find out," Obama said. "We will find whoever harmed our citizens and we will bring them to justice. We also know this: the American people refuse to be terrorized."