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Obama: We don't know who was behind 'heinous, cowardly attack'

President Obama on Tuesday called the bombing of the Boston Marathon an act of terrorism as authorities sought to figure out who was behind the attack that killed three people and injured 176. 

“This was a heinous and cowardly act, and given what we now know took place, the FBI is investigating it as an act of terrorism,” Obama said in remarks from the White House press room. “Any time bombs are used, it is a terrorist act."

Obama then echoed federal, state and local authorities in making it clear that the government does not know the motive behind the attack, or whether it was conducted by a foreign or domestic group or individual. 

"What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual," Obama said. 

New details were revealed Tuesday about the makeup of the two bombs that exploded near the finish line of the marathon. Authorities in Boston said two crude bombs were responsible for the devastation, and that initial reports suggesting that more devices were found were incorrect.

On Tuesday evening, Rick DesLauriers, the special agent in charge of the FBI's Boston division, said clues recovered from the crime scene indicated that the bombs were carried in a black nylon backpack and were made of a pressure cooker device and metal shards.

"Among items partially recovered are pieces of black nylon which could be from a black backpack and what appear to be fragments of BBs and nails possibly contained in a pressure cooker device," DesLauriers said. "In addition, this morning, it was determined that both of the explosives were placed in a dark colored nylon bag or backpack."

"The bag would have been heavy because of the components believed to be in it. At this point it is difficult to determine specific components used until we can eliminate other factors which may have already been present in the environment," he said. 

In his comments, Obama largely echoed federal, state and local law enforcement officials who on Tuesday were tight-lipped about any leads they might have. The officials also refused to say whether anyone was in custody

“I'm not going to say who might be in custody right now,” DesLauriers said. 

He and other officials said there were no imminent threats to Boston but promised an active police presence. Officials also asked the public to come forward with any video or photographs that might help with the investigation, with DesLauriers saying such assistance was critical in establishing a timeline of events.

Obama met Tuesday with his counterterrorism team, including Homeland Security Secretary Janet Napolitano, Attorney General Eric Holder, FBI Director Robert Mueller, and counterterrorism aide Lisa Monaco.

Obama said that it will “take time to follow every lead” but pledged that ”we will find whoever harmed our citizens and bring them to justice.”

"If you want to know who we are, what America is, how we respond to evil, that's it: selflessly, compassionately, unafraid," Obama said, adding "the American people refuse to be terrorized."

Obama also ordered Tuesday that all flags on U.S. government buildings be lowered to half-staff to honor the victims.

Lawmakers on the Senate’s Intelligence panel were expected to receive a briefing later Tuesday. Security at the Capitol has been tightened, with some visitors asked to take off their shoes before entering some buildings.

"It's important to clarify that two and only explosive devices were found yesterday," Massachusetts Gov. Deval Patrick (D) said. "Other parcels have been examined but there are no unexploded bombs. There were no unexploded devices found.”

Patrick was flanked by the lead agents in the investigation for the FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives, as well as Boston Mayor Tom Menino (D) and Massachusetts Sens. Elizabeth Warren (D) and Mo Cowan (D).

Boston Police Commissioner Ed Davis said updated casualty figures indicate three people were killed and 176 were hurt by the bombings. He said 17 of those injured are in critical condition.

Patrick also said there would be an inter-faith prayer service Wednesday in Boston. Obama ignored shouted questions about whether he would attend, and a White House official said he was not yet aware of any travel plans.

One of the three people killed in the bombings has been identified as an 8-year-old boy.

The father of 8-year-old Martin Richard released a statement Tuesday confirming his death. 

"My wife and daughter are both recovering from serious injuries," Bill Richard, Martin's father, said in the statement. "We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin. We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you."

Krystle Campbell was identified as the second victim who died in the bombing, according to the Boston Globe.

This story was posted at 10:38 a.m. and was updated at 6:09 p.m.