Obama cautioned that authorities were still investigating and that people should not jump to any conclusions before all of the facts are learned.
“But make no mistake,” Obama said. “We will get to the bottom of this and we will find out who did this. We’ll find out why they did this.”
Individuals and groups responsible “will feel the full weight of justice,” the president said.
“We will find out who this and we will hold them accountable,” he said.
1 person questioned in Boston Marathon explosions | 2:59 p.m.
Officials began questioning at least one person in connection with the explosions that tore through the finish line of the Boston Marathon on Monday, killing at least two people and injuring dozens more, officials said.
Meanwhile, a third explosion was reported, but investigators later said it was just fire-related and unconnected to what was increasingly looking like a deliberate attack on one of the nation’s most celebrated sporting events.
A federal law enforcement official said authorities were questioning a Saudi national who was taken to a Boston hospital with injuries. The person was not identified as a suspect.
The official also said authorities are also “desperately seeking” a Penske rental truck seen leaving the race site. They further believe the explosive devices were small bombs placed in small receptacles and that at least one was detonated in a nearby trash can.
The third explosion was at the JFK Library; no additional injuries were reported, Police Commissioner Edward Davis said during a nationally broadcast news conference. Officials later said the library blast appeared to be fire-related.
The police chief urged people to stay indoors and not congregate in large groups, casting an ominous pall over what should have been a celebratory scene after the annual running of marathon. About 27,000 runners took part in the 26.2-mile race, one of the world's premier marathons.
The situation remained fluid as officials tried to count the injured. Earlier, they had indicated that 23 were hurt by the blasts along Boylston Street near the finish line of the marathon. But Davis said it was too soon to confirm any numbers.
Reports from the scene, however, put the number of injured at as many as 100 people, some of whom had limbs torn off, according to eyewitnesses.
In major cities across the nation, officials went on high alert after the explosions.
“Out of an abundance of caution, we have expanded our security perimeter at the White House complex. It is not unusual to expand or contract these security perimeters,” Secret Service spokesman Ed Donovan said.
Officials were treating the incident as a deliberate bombing, though there no immediate indication of a reason or a suspect.
Officials in Boston and Washington, D.C., agreed that there had been no warning of the tragedy.
“There was, nothing, nothing on the radar screen,” in terms of terrorist threat information, a U.S. homeland security official said.
The deadly blasts took place within seconds of each other and about 100 yards apart.
“This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the president, Mayor [Thomas] 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,” Massachusetts Gov. Deval Patrick said.
President Obama is expected to address the nation Monday evening Eastern time.
Chaos erupts at Boston Marathon; 2 dead | 1:53 p.m.
At least two explosions shattered the festive air at the end of the grueling Boston Marathon on Monday, leaving at least two people dead and injuring as many as 23, officials said.
About three hours into the 117th running of the 26.2-mile race, an explosion went off at the Boylston Street finish line, witnesses said, and that blast was followed by a second explosion just seconds later.
Video images showed bloody spectators looking dazed and chaos erupting as runners quickly shuffled to the side and would-be rescuers surged forward. Ambulances and firefighters poured into the scene, rushing the victims to hospitals.
Cellphone service crashed, and nearby hotels went into lockdown mode.
“It’s chaos here,” said spectator Samantha Bissonnette, a 22-year-old senior at Tufts University. In texts, she said she was about half a mile away when the explosions went off, and described hearing two claps “as loud as thunder.”
“I thought they were taking the stands down. ... It was so loud, I knew it couldn’t be thunder. ... Not a cloud in the sky. Then slowly they blocked off the runners ... cops ran in packs down the street. The runners seemed confused and slowly panic started to set in ... people used their cheering signs to find their families.
“I saw pictures of the blast on twitter ... everyone was passing phones to runners to contact their families.”
Kayley Pettoruto, 23, a graduate student at Boston University, said she went to the marathon to support former track teammates. She said she was about a mile away when she heard the sirens.
"I was really shocked,” she said. “Freaked out a little. It’s the Boston Marathon; you’d never think this would happen. At first, I thought maybe it was a gas explosion.”
Will Ritter, communications director for Massachusetts U.S. Senate candidate Gabriel Gomez, said he was about a block away from the grandstands near the finish line when he heard two loud explosions, followed by billowing white smoke.
Ritter said that when the loud bangs occurred, he had been setting up a news conference with Gomez, a former Navy SEAL who had just finished the marathon about minutes earlier.
The blasts were seconds apart, he said, about an hour after the first male contestants crossed the finish line.
Officials immediately launched an investigation, but racer organizers referred to "bombs" to explain the explosions.
In Washington, President Obama was told of the incident.
“The president has been notified of the incident in Boston. His administration is in contact with state and local authorities. He directed his administration to provide whatever assistance is necessary in the investigation and response,” a White House official said.
Shortly after being notified around 3 p.m. local time, the president got a briefing in the Oval Office from homeland security advisor Lisa Monaco and other members of his senior White House staff, the White House said.
Obama then called Boston Mayor Tom Menino and Massachusetts Gov. Deval Patrick to express his concern for those who were injured and to make clear that his administration is ready to provide needed support as they respond to the incident, officials said.
Vice President Joe Biden interrupted his remarks on a telephone call with gun control advocates after an aide turned on the television, reports said.
“Apparently there has been a bombing. I don’t know any of the details of what caused it,” he said. “Our prayers are with those people in Boston.”
This story was reported by Jessica Garrison, Maeve Reston, Matt Pearce, Michael Muskal, Christine Mai-Duc, Julie Cart and Rosanna Xia in Los Angeles. Also contributing were Christi Parsons, Matea Gold, Kathleen Hennessey, Ken Dilanian and Michael A. Memoli in Washington. Alana Semuels contributed from New York.
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