Two bombs exploded in the packed streets near the finish line of the Boston Marathon on Monday, killing three people and injuring more than 140 in a terrifying scene of shattered glass, bloodstained pavement and severed limbs at the world's oldest and most prestigious marathon.
A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile (42-kilometer) course.
At the White House, President Barack Obama vowed that those responsible will "feel the full weight of justice."
A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.
There was no word on the motive or who may have carried out the attack, and police said they had no suspects in custody. Authorities in Washington said there was no immediate claim of responsibility.
A senior U.S.intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.
"They just started bringing people in with no limbs," said runner Tim Davey, of Richmond, Virginia. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."
"They just kept filling up with more and more casualties," Lisa Davey said. "Most everybody was conscious. They were very dazed."
The Anti-Defamation League (ADL) expressed "shock and horror" at the terrorist attack and cited the anniversaries of domestic terror incidents in the U.S. that took place this week.
ADL National Director Abe Foxman said in a statement: "This apparent terrorist attack comes during a week when we are already on heightened alert because of the history of extremist-related events that have taken place during the week of April 20th, including the Oklahoma City bombing and the federal raid of the Branch Davidian Compound in Waco, Texas. "
Foxman said that due to the anniversary of Adolph Hitler's birthday, "ADL issued a security alert to Jewish community institutions reminding them to be on high alert during this time of year."
Alasdair Conn, chief of emergency services at Massachusetts General Hospital told Haaretz: "This is like a bomb explosion we hear about in Baghdad or Israel or other tragic points in the world."
He also said that a few years ago a team of Israeli doctors came to the Hospital to help them set up a "disaster team" to deal with this type of bombing scenario.
"A few years ago we wanted to upgrade our emergency response to things like explosions, and unfortunately Israel was dealing with several types of [these incidents] a year... and we had to upgrade our response."
"This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population," he said. "This is what we expect from war."
The fiery twin blasts took place almost simultaneously about 100 meters apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the course.
When the second bomb went off, the spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.
A pool of blood formed, and huge shards were missing from window panes as high as three stories.
Boston police said three people were killed. Hospitals reported at least 134 injured, at least 15 of them critically. The injuries ranged from cuts and bruises to amputations. Many victims suffered lower leg injuries and shrapnel wounds. Some suffered ruptured eardrums.
Some 23,000 runners took part in the race, which attracts more than 500,000 spectators and winds up in the heart of central Boston, near the landmark Prudential Center and the Boston Public Library. It is held on Patriot's Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775.
Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Obama also told Boston Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed, the White House said.
"We still don't know who did this or why," Obama said, adding, "Make no mistake: We will get to the bottom of this."
At Congress, members of intelligence committees said they expected to be briefed on the attack on Tuesday.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know precisely where the bombs were planted or whether they were hidden in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
Across the U.S., security was tightened at landmarks, government buildings, transit hubs and sporting events. Law enforcement agencies urged the public, via Twitter and Facebook, to report suspicious activity to the police.
In New York, authorities deployed so-called critical response teams- highly visible patrol units that move in packs- along with more than 1,000 counterterrorism officers. Highly trafficked areas like the Empire State building, Rockefeller Center, St. Patrick's Cathedral, the United Nations and the World Trade Center site were being especially monitored.
Worries also reverberated across the Atlantic. In London, an already robust security operation was being beefed up ahead of the ceremonial funeral that will be held for former Prime Minister Margaret Thatcher. The event, which will be attended by Queen Elizabeth II and many other world dignitaries, will include a procession through the city's streets. British police were also reviewing security plans for the London Marathon, which is to be held on Sunday.
Boston Marathon Terror Attack
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