President Barack Obama says the explosions at the Boston Marathon are being investigated as an act of terror, although authorities still don't know who is responsible.

He called the bombing "a heinous and cowardly act" used to target innocent civilians.

Obama spoke to reporters at the White House after a briefing by his national security team.

Three people were killed, including an 8-year-old boy, and more than 170 were wounded in Monday's bombing at the famous marathon's finish line.