WASHINGTON (AP) -- 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.
WASHINGTON (AP) -- Attorney General Eric Holder has directed the full resources of the Justice Department be deployed to investigate the bombs that exploded at the Boston Marathon.
A department official said Holder has spoken with FBI Director Robert Mueller (MUHL'-ur) and with Carmen Ortiz, the U.S. attorney for Massachusetts. The official said Ortiz's office was coordinating the department's response with the FBI and other federal, state and local law enforcement authorities.
The official spoke on condition of anonymity because she was not authorized to speak on the record.
Two bombs exploded near the marathon's finish line on Monday, killing two people and injuring many others. A senior U.S. intelligence official said two other explosive devices were found nearby. A third explosion followed at the JFK Library in Boston.
(Photo courtesy of ABC News)
Boston, MA - Two explosions erupted the finish line of the Boston Marathon today, turning the annual race into a bloody crime scene.
The blasts occurred on Boyleston Street about three hours after the top runners had finished the race, but with thousands of marathoners still running the route.
According to law enforcement sources, the first blast was at the Marathon Sports running store before 3 p.m., and blew out windows in four nearby buildings, injuring 15 to 20 individuals.
About 10 second later, a second explosion occurred, severely injuring more bystanders, police said.
Boston EMS personnel could be seen shuttling the injured out of the blast area on wheelchairs. Several of them were bleeding from the face.
Massachusetts Genreal Hospital has received four patients and is expecting more, according to a hospital spokesman.
A trauma nurse from the hospital told ABC News that the race's medical tent, already set up to deal with runner injuries, set up a morgue area at a triage center at the back of the tent. The medical staff reported multiple victims with severed limbs and children with severe burns.
Bomb squads are sweeping the area, checking suspicious packages and are testing for chemicals to help determine what kind of device was used, according to police.
Homeland Security Direction Eric Holder was in touch with the FBI in Boston and President Obama was notified of the blasts.
Police have told people in area to avoid trash cans, according to witnesses.
The explosions erupted on what is usually a festive day in Boston. It is designated Patriots Day and most offices are closed for the celebration and the marathon.
Debris from the explosions could be seen scattered throughout the spectators stands and finish line area of the marathon as emergency personnel cleared the area.
Video of the explosions showed plumes of white smoke pouring into the air above the street where runners were.
More than 26,00 runners were registered to compete in this year's marathon.