President Obama promised those responsible for the deadly bombings at the Boston
Marathon Monday would feel the "full weight of justice."
Speaking at a news conference at the White House, Obama said Americans "stand with the people of Boston" and would pray for the victims and their families.
"We still do not know who did this or why, and people should not jump to conclusions before we have all the facts," Obama said. "But we will find out who did this and we'll find out why they did this. The responsible individuals and groups will feel the full weight of justice.
"We will find out who did this and we will hold them accountable."
Obama left the podium without taking questions.
Boston's police commissioner said there were no warnings before two explosions ripped through crowds at the marathon, killing at least two people. WCVB-TV, Boston, reported one of the dead was an 8-year-old.
Media reports put the injured at 50-100, but the death toll and number of injured was expected to grow. ABC News said it had confirmed 86 people injured by calling hospitals and The Boston Globe reported 107 were injured.
The president provided no hard numbers of the dead and wounded but said "we do know that multiple people were wounded, some gravely."
Obama said he had updated leaders of both parties in Congress and offered Boston and Massachusetts officials whatever federal resources they need to deal with the tragedy.
He said emergency workers and others "responded heroically and continue to do so as we speak without regard for their own safety."
"We salute all those who assisted and responded so quickly to this tragedy," Obama said.
"Boston is a tough and resilient town and so are its people. I'm confident they will pull together and move forward. The American people will be with them every single step of the way."
Police Commissioner Edward Davis, at a news conference, indicated officials had found a third device and detonated it, but did not give details.
Davis later backed off an earlier report that there may have been a fourth incident involving a fire at the JFK library, saying it appeared "premature" to link it to the bombings.
Local reports said the fire in a mechanical room at the John F. Kennedy Presidential Library and Museum in the Dorchester neighborhood of Boston was quickly tamped down, but officials were investigating.
The two explosions at the Boston Marathon "occurred 50 to 100 yards apart" and "resulted in multiple causalities," Davis said.
"All the casualties have been removed from the scene," he said, but racers and spectators fleeing the scene left behind a large number of backpacks and other types of parcels, and investigators were methodically searching each one.
Asked if there were any warnings before the explosions, Davis said, "None."
Massachusetts Gov. Deval Patrick said he received a phone call from Obama after the explosions, promising the full effort of the FBI.
A White House official said Obama received briefings from FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano on the active investigation and response to the incident, including the ongoing coordination with state and local officials.
At the Boston news conference, Davis said families of the victims could call a city line at 617-635-4500. Witnesses who want to talk to officials could call 800-494-TIPS.
He said officials did not yet know how many victims there were.
"People should remain calm but they should understand this is an ongoing event," Davis said. He asked people in Boston to stay at home and if they are visiting for the race, to return to their hotels and not congregate in large crowds.
Davis stopped short of calling the explosions bombs but said people can "draw your own conclusions."
Security has been tightened in New York and Washington as a precaution.
"This is a horrific day in Boston," Patrick said earlier.
"My thoughts and prayers are with those who have been injured," Patrick said in a statement. "I have been in touch with the president, Mayor [Thomas M.] Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs."
The first two blasts, which were about 15 seconds apart, occurred around 2:50 p.m., near the intersection of Boylston and Exeter streets where thousands of people were lined up to watch the 117th Boston Marathon.
The area was evacuated. The explosions occurred about 3 hours after the winners crossed the finish line. Video showed people trapped under scaffolding in the area.
The Boston Marathon follows a 26.2-mile course that winds through downtown Boston and outlying cities. An estimated 500,000 people come out to watch the race each year, The New York Times said.
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