OBAMA: Boston Marathon Explosions Are Being Investigated As An 'Act Of Terror'

President Barack Obama confirmed Tuesday that the Boston Marathon explosions are being investigated as an "act of terror," addressing members of the press one day after the fatal blasts that killed three people and injured more than 170.

"This was a heinous and cowardly act," Obama said. "The FBI is investigating this as an act of terrorism. … We don’t yet know who carried this out or why."

The statement marked the first time that Obama characterized the attack as terrorism. On Monday, the President urged the nation not to "jump to conclusions" about the attacks, but promised that whomever was responsible would "feel the full weight of justice."

He reiterated this sentiment Tuesday in stronger terms.

"The American people refused to be terrorized," Obama said, noting the acts of courage by first responders and Good Samaritans in Boston.

"If you want to know who we are, what America is, how we respond to evil — that’s it — selflessly, compassionately, unafraid."

Earlier Tuesday, Obama ordered flags to be lowered to half-mast until April 20 in honor of the Boston victims.