WASHINGTON (AP) — A stony-faced President Barack Obama declared that those responsible for the explosions at the Boston Marathon "will feel the full weight of justice," but he urged a nervous nation not to jump to conclusions. Top lawmakers declared the deadly incident an act of terrorism, and a White House official said it was being treated that way. Obama, speaking from the White House late Monday, pointedly avoided using the words "terror" or "terrorism," saying officials "still do not know who did this or why."