President Obama says those responsible for marathon bomb will ``feel full weight of justice’’

WASHINGTON – President Obama on Monday evening pledged the full weight of the federal government to figure out who is responsible for Monday’s double bombing at the finish line of the Boston Marathon, and the White House said it is treating the attack as an ``act of terror.’’

In a three-minute address from the White House, Obama urged caution, saying “people shouldn’t jump to conclusions before we have all the facts.”

Significantly, reflecting caution about the unknown motive and perpetrators, the president himself did not call the incident a terrorist attack. However, a White House official speaking after the president’s remarks but only on the condition of anonymity, said that the administration views it that way.

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“Any event with multiple explosive devices—as this appears to be—is clearly an act of terror, and will be approached as an act of terror,” a White House official said. “However, we don’t yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic.”

Washington put aside its differences as a steady stream of sympathy and concern flowed for Boston and the racers in the marathon.

“Make no mistake,” Obama said in his remarks. “We will get to the bottom of this. And we will find out who did this, we’ll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice.”

The president noted that the explosions occurred on Patriots Day, “a day that celebrates the free and fiercely independent spirit that this great American city of Boston has reflected from the earliest days of our nation.”

“Boston is a tough and resilient town,” Obama added. “So are its people. I’m supremely confident that Bostonians will pull together, take care of each other, and move forward as one proud city. And as they do, the American people will be with them every single step of the way.”

Obama was notified soon after the explosions occurred and he quickly contacted Governor Deval Patrick and Boston Mayor Thomas M. Menino. The White House released a photo of the president receiving a briefing.

He spoke with Homeland Security adviser Lisa Monaco, FBI Director Robert Mueller, and Department of Homeland Security Secretary Janet Napolitano on the active investigation and response to the incident, including the coordination with state and local officials, according to a White House official.

Moments of silence were held on the House and Senate floors, flags at the Capitol were lowered to half-staff, and politicians quickly sent out statements and tweets reacting to the news. The partisan chill enveloping Washington was, at least for the moment, suspended. Obama spoke with House Speaker John Boehner by telephone at 5:30 p.m., and Boehner’s office released a photograph of the speaker on the phone during the conversation.

Senator Pat Toomey, a Pennsylvania Republican, informed the Senate of the explosions shortly after they happened.

“It appears that a tragedy has struck at the Boston Marathon,” he said. “Bombs have gone off and there are injuries we know of…very, very disturbing news.”

Vice President Joe Biden interrupted a conference call on gun control, saying that an aide had turned televisions to the news and “apparently there has been a bombing.”

“I don’t know any of the details of what caused it, who did it,” Biden said on the call. “Our prayers are with those people in Boston who have suffered injuries. And I don’t know how many of them there are.”

Within 90 minutes of the explosions, Representative Edward J. Markey announced he would suspend all campaign activities in the Democratic primary for the US Senate seat vacated by John F. Kerry. His opponent in the Democratic primary, Stephen F. Lynch, suspended his campaign soon afterward, as did former US Attorney Mike Sullivan, who is running in the Republican primary.

“The heart of the city is hanging heavy,” Markey said in a statement.

Lynch said “no matter who or what is responsible, this is a terrible tragedy for our city and our nation.”

Sullivan added that “our hearts and prayers are with those injured in today’s horrific explosion during the Boston Marathon and the families of all concerned.”

Senator William “Mo” Cowan, the interim from Massachusetts, quickly made plans to leave Washington and head home to Boston.

Secretary of State John Kerry was informed of the explosions by a senior aide as his plane approached O’Hare International Airport after a 10-day trip abroad, according to a State Department official.

He contacted his youngest daughter, Vanessa, a doctor in Boston who has run the marathon in previous years, to confirm that family and friends were safe. Kerry had noted on Monday morning that it was one of the few years he wasn’t on hand to fire the starting gun for the wheelchair race.

As Kerry flew back to Washington later in the day, he called Patrick to check in on the response and investigation.

Representative William R. Keating, a member of the Homeland Security, said his office had been receiving information throughout the day, though no formal briefing.

“There is clearly a coordinated, sophisticated attack that has taken place, the nature of which is not clear now,” Keating said.

He said he had no direct knowledge of motives, but noted that in addition to Patriots Day, it was also tax day, which could inspire domestic terrorists.

Representative Joseph Kennedy III tweeted: “Heartbreaking day for Boston. Praying for everyone back home as this terrible tragedy unfolds.”

Senator Elizabeth Warren wrote a simple tweet: “Praying for those at the Boston Marathon today.”

Mitt Romney, the former Massachusetts governor and 2012 Republican presidential nominee, also sent a note on twitter. “Our hearts are heavy with the news out of Boston today,” he wrote, adding with a hashtag, “PrayforBoston.”