President Obama on Tuesday morning said it was still not clear who was responsible for a pair of explosions that killed three people and injured dozens more at the Boston Marathon but vowed his administration would “get to the bottom of what happened.”

“This was a heinous and cowardly act, and given what we know about what took place, the FBI is investigating it as an act of terrorism,” Obama said from the White House briefing room. “Anytime bombs are used to target innocent civilians, it is an act of terror. 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.”

It was the first time the president publicly labeled the violence an act of terrorism.

“We know that two explosions gravely wounded dozens of Americans and took the lives of others, including an 8-year-old boy,” Obama said.

And the president pressed for the public to come forward if they had any information that could help authorities crystalize what happened near the finish line of the Boston Marathon on Monday.

“If you see something suspicious, speak up,” the president said.

Amid the tragedy, Obama praised the first responders, runners and volunteers in the New England city for rushing to the aid of the victims.

“We also know this: 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.

Following his remarks, Obama did not respond to questions shouted by reporters. He said officials would provide more briefings throughout the day as information becomes available.