President Obama speaks at the White House on Tuesday about the Boston Marathon explosions. (Susan Walsh/AP)

President Obama told reporters Tuesday morning that federal authorities are investigating Monday’s bombings at the Boston Marathon “as an act of terrorism,” though they still did not know who perpetrated the attack and what might have been the motive.

“Any time bombs are used to target innocent civilians, it is an act of terror,” Obama said in the White House briefing room. “What we don’t yet know, however, is who carried it out, and why.”