President Barack Obama said Tuesday the deadly Boston Marathon bombings were an act of terrorism but investigators do not know if they were carried out by an international or domestic organization, or perhaps by a "malevolent individual."

In his second public statement in less than 24 hours since the explosions, the president said, "Clearly we are at the beginning of our investigation." He urged anyone with information relating to the events to contact authorities.

Obama said investigators "don't have a sense of motivation yet" as they begin to evaluate the attack in which three people were killed, including an 8-year-old boy, and more than 170 were wounded.