"This was a heinous and cowardly act and given what we now know about what took place, the FBI is investigating it as an act of terrorism," Obama said at Tuesday's press conference. "Anytime bombs are used to target innocent civilians, it is an act of terror."
The President also said investigators "don't have a sense of motivation yet" as they begin to evaluate the attack in which three people were killed, including Martin Richard, an 8-year-old boy, and more than 170 were wounded.
"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," Obama said.
"It will take time to follow every lead and determine what happened. But we will find out. We will find whoever harmed our citizens and we will bring them to justice," th President promised.
It was Obama's second public statement in the 24 hours following the attack.
Individuals briefed on the probe said the two bombs were made up of pressure cookers, one packed with ball bearings and the other with shards of metal, presumably designed to inflict maximum injuries.
The explosions occurred near the finish line at Monday's Boston Marathon, near 671 Boylston on opposite sides of the street near Copley Square around 2:50 p.m., about halfway through the race.