Obama on Boston attacks: Those responsible will feel ‘weight of justice’

President Barack Obama vowed Monday to bring the "full weight of justice" down on those behind the apparent bombing attacks in Boston. Obama, speaking at the White House, stopped short of explicitly blaming terrorists for the dramatic blasts that ripped through a crowd near the finish line of the Boston Marathon. But a White House official told reporters the incident would be treated as "an act of terror."

“We still do not know who did this, or why, and people shouldn’t jump to conclusions before we have all the facts," Obama said in brief prepared remarks delivered in the briefing room. "But make no mistake: 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," Obama emphasized.

"The American people will say a prayer for Boston tonight. And Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss," he said. Officials said the attack left at least two dead and some 80 injured.

Earlier, the president spoke by telephone with Republican House Speaker John Boehner, Democratic Senate Majority Leader Harry Reid and Republican Senate Minority Leader Mitch McConnell.

"We reaffirmed that, on days like this, there are no Republicans or Democrats: We are Americans united in concern for our fellow citizens," Obama said.

The president also received briefings on the crisis from FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano, and telephoned Boston Mayor Tom Menino and Massachusetts Gov. Deval Patrick to express concern. He also offered Washington's help in the investigation and response to the incident.

“We’re continuing to monitor and respond to the situation as it unfolds, and I’ve directed the full resources of the federal government to help state and local authorities protect our people, increase security around the United States as necessary, and investigate what happened,” Obama said.

A White House official, speaking on condition of anonymity, told reporters by email that “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.”

“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,” the official said.

Vice President Joe Biden, speaking earlier on a conference call with gun control advocates, reacted to television images of blood and panic on the streets of Boston, saying "apparently there has been a bombing."

"I don’t know any of the details," Biden said in a shocked voice. "Our prayers are with those people in Boston who have suffered injury. I don’t know how many there are."

The vice president was reacting solely to the images on television and his comments did not reflect an official finding of the causes of the carnage in Boston, an aide said.

"The president has been notified of the incident in Boston," a White House official said earlier on condition of anonymity. "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."

At the White House, authorities used yellow police tape to block off Pennsylvania Avenue that runs right outside the White House gates. Secret Service moved to secure the expanded perimeter, while credentialed pass holders were directed to exit through the adjacent Eisenhower Executive Office Building gates, not the usual White House gates.

"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," U.S. Secret Service spokesman Edwin Donovan said.