In a brief statement, President Obama vowed to find the person or people responsible for the Boston Marathon bombing and urged Americans not to "jump to conclusions before we have all the facts."
"We still do not know who did this or why," Obama said in a White House press briefing.
Obama said "every federal resource necessary" will be used to find those behind the attack and to help those who were injured. Three were killed and more than 90 injured after two explosions at the Boston Marathon finish line.
"Make no mistake, we will get to the bottom of this and we will find out who did this and we will find out why they did this," the president said. "Any responsible groups will feel the full weight of justice."