WASHINGTON - US President Barack Obama, responding to the explosions at the Boston Marathon, says the United States does not know "who did this or why" but vowed that whoever is responsible "will feel the full weight of justice."

He said: "We will find out who did this and we will hold them accountable."

Obama made his remarks Monday evening from the White House about three hours after two explosions detonated near the marathon's finish line. At least two people were killed and 50 injured in the blasts.

Obama has been in touch with federal law enforcement and Massachusetts officials in the aftermath of the explosions.

The Secret Service reacted cautiously to the blasts, expanding the security perimeter around the White House.