The FBI was investigating Monday's explosions at Boston Marathon finish line as "an act of terrorism," U.S. President Barack Obama said on Tuesday at the second televised statement following Boston explosions.

Obama called the explosions, which killed at least three and injured 144, "a heinous and cowardly act."

"And given what we now know about what took place, the FBI is investigating it as an act of terrorism," said Obama, "Any time bombs are used to target innocent civilians, it is an act of terror."

Obama said that the authorities does not know "who did this or why," but vowed that whoever is responsible "will feel the full weight of justice."

Obama said the federal government will continue to mobilize and deploy all appropriate law enforcement resources to the investigation and response work.

Two bombs struck near the finish line of the Boston Marathon on Monday, turning a celebration into a bloody scene of destruction. Police said a third explosion hit the Boston JFK Library.

The blasts killed three and injured 180. The dead were?Martin Richard, an 8-year-old boy, Krystle Campbell, a 29-year-old Massachusetts woman and a Chinese graduate student at the Boston University whose name has not been identified, pending permission from the family.