President Obama vowed today that whoever is responsible for the bombing that killed at least three people at the Boston Marathon "will feel the full weight of justice."
"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this, and we will find out who did this," Obama said, offering condolences to victims and their families and praising first responders.
"We will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice," Obama said.
The president was briefed within minutes of the explosions that killed at least three people and injured more than 130 more near the finish line of the marathon, as the federal government boosted security measures and offered pledges of law enforcement support to the investigation.
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Senators on the Intelligence and Homeland Security Committees, meanwhile, said the explosions had the hallmarks of a terrorist attack, either domestic or international.
Read the ABC News Live Blog of the Explosions at Boston Marathon
Shortly after the two blasts near the marathon finish line, and as reports of injuries flowed from local sources in Boston, Washington's security apparatus began taking precautions. At the White House, Pennsylvania Avenue was closed to tourists, who remained across the street in Lafayette Park.
Homeland security advisers briefed Obama on the explosions around 3 p.m., according to a White House official. The president is tracking the case with his homeland security team.
Obama spoke with House Speaker John Boehner around 5:30 p.m., according to a Boehner aide.
The president called Boston Mayor Tom Menino and Massachusetts Gov. Deval Patrick to express concern for the injured and notify them that the White House is ready to supply any needed support to the city and state.
Patrick announced at a press conference shortly before 5 p.m. that FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives agents were in Boston.
All of the ABC News Photos of the Boston Marathon Explosions
Vice President Biden was the first high-ranking administration official to publicly react. The VP was on a conference call with gun-control advocates when he first heard of the incidents.
"As I am speaking here they just turned on the television in my office, and apparently there has been a bombing, I don't know any of the details of what caused it, who did it, I don't think it exists yet. But our prayers are with those people in Boston, who have suffered injury," Biden said on the call. "I am looking at it on television now."
Attorney General Eric Holder spoke with FBI Director Robert Mueller and U.S. Attorney Carmen Ortiz this afternoon and directed the Justice Department to ensure the incident is fully investigated, a department official told ABC News. The U.S. Attorney's Office for the District of Massachusetts is coordinating the department's response with FBI, ATF, and other federal, state, and local authorities, the official said.
According to a White House official, any event with multiple explosive devices will be considered an act of terrorism, but the official warned that an investigation has yet to determine whether it was planned or carried out by a terrorist group. The National Nuclear Security Administration did detect any radiological material involved in the explosion, according to an official.
On the House floor, Rep. Darrell Issa, R-Calif., requested a moment of silence to recognize the loss of life in Boston shortly after 5 p.m. on Monday. House Speaker John Boehner held another moment of silence just before 7 p.m.
"Whatever it was, it was a terrible tragedy ... no matter how you measure it," said Rep. Michael Capuano, D-Mass., who represents Boston.
Boehner yielded the floor to Capuano before the moment of silence began.
"Clearly anyone to act in such a manner is an evil person and deserves to be called as such," Capuano said.
The Senate held its own moment of silence at 5:38 p.m. Senate Majority Leader Harry Reid, D-Nev., said beforehand that he was "shocked and saddened" by the news.
"My thoughts go out to all those who were injured, and my condolences go to the families and friends of those affected by this tragedy. I commend the first responders, observers who rushed toward danger, to help those who were hurt," Reid said. "I'll do whatever I can to support the people of Boston and the commonwealth of Massachusetts as we all will, all will during this difficult time."
Massachusetts lawmakers expressed their concern after hearing of the explosions.
"Monitoring the scene back home. My thoughts and prayers are with the injured and everyone at @bostonmarathon, in Boston and in Mass," Massachusetts Sen. Mo Cowan tweeted.
Suspending his special-election campaign activities Democratic Senate candidate Ed Markey said he was "disturbed" by the news.
"I am disturbed and saddened by the explosions at the site of the Boston Marathon this afternoon. We all are grateful for the first responders who rushed to the scene to help the victims," Markey said in a statement released to reporters. "The heart of the city is hanging heavy, and our thoughts and prayers are with everyone affected by this senseless tragedy."
Sen. Dianne Feinstein, who chairs the Senate Intelligence Committee, said she felt there was no doubt the explosions were an act of terrorism.
"It could be foreign, it could be homegrown," Feinstein told ABC News in an interview at the Capitol. Feinstein said the incident in Boston had all the "hallmarks" of a terrorist attack. The senator said she has been briefed by intelligence officials and would receive a formal briefing tonight.
Senators Susan Collins and Angus King of Maine compared the explosion to an act of terrorism.
"As members of the Senate Intelligence Committee, we will be continually updated of the situation. In the meantime, initial press reports that multiple improvised explosive devices may have been involved at this high profile national event bear the hallmarks of a terrorist attack," the senators wrote in a joint statement.
Senator Mary Landrieu of Louisiana, a member of Homeland Security Committee, told ABC News that she and her colleagues on the committee were expecting to receive a briefing tonight on Boston.
She said "it's too soon" to speculate on who is responsible.
Authorities in Boston, Philadelphia, and Washington, D.C., were on high security alert after the explosions.
The District of Columbia's public transit system, Washington Metropolitan Area Transit Authority (WMATA) stepped up security measures spurred by what they called "an abundance of caution."
WMATA encouraged riders on Twitter to call Transit Police (202-962-2121) if they noticed anything suspicious in the system.
A release from WMATA said that Metro Transit Police Chief Ronald Pavlik asked all officers serving during the day stay on-duty through the rush-hour period in the evening.
"We're going to continue to monitor the events in Boston and we'll take appropriate measures," said WMATA public information officer Philip Stewart. "If heightened security is appropriate we will make that measure."