Susan Walsh/AP Photo

President Obama today said the bombings at the Boston Marathon are being investigated as an "act of terrorism" and vowed that those responsible for the attack would be brought to justice.

"This was heinous and cowardly act, and given what we now know about what took place, the FBI is investigating it as an act of terrorism. Any time bombs are used to target innocent civilians, it is an act of terror," the president told reporters at the White House shortly after being briefed by his national security team.

The president's comments come 17 hours after he first publicly addressed the attack, describing it as the "events in Boston" and declining to categorize it publicly as an act of terror or terrorism.

Authorities are still working to determine who was behind the attack and investigators "don't have a sense of motive yet," 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," he said. "Clearly we're at the beginning of our investigation. 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."

The president praised those who aided the victims. "The American people refuse to be terrorized because what the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love," he said. "If you want to know who we are, what America is, how we respond to evil, that's it: selflessly, compassionately, not afraid."