WASHINGTON » President Barack Obama pronounced the deadly Boston Marathon explosions an act of terrorism today as individuals close to the investigation said the two bombs were made of pressure cookers packed with ball bearings and metal shards that cut into the victims.

Speaking at the White House, Obama said investigators do not know if the attack was carried out by an international or domestic organization, or perhaps by a "malevolent individual." Three people were killed, including an 8 year-old boy, and more than 170 were wounded.

In his second public statement in less than 24 hours since the explosions, the president said, "Clearly we are at the beginning of our investigation." He urged anyone with information relating to the events to contact authorities.

Individuals briefed on the probe said the two bombs were made up of pressure cookers, one packed with ball bearings and the other with shards of metal, presumably to inflict maximum injuries. The bombs were placed inside black duffel bags on the ground near the finish line of the annual race, they said, speaking on condition of anonymity because the investigation remains active and they were not authorized to be quoted by name.

Obama said investigators "don’t have a sense of motivation yet" as they begin to evaluate the attack.

Despite the loss of life and limb, Obama declared, "The American people refuse to be terrorized."

As he had on Monday, he said those responsible for the attacks would be brought to justice.

The president had avoided labeling the incident a terrorist attack when he stood at the same White House lectern shortly after the explosions. Members of Congress quickly concluded on Monday afternoon that’s what it was, and White House officials said the FBI was investigating the attack as a terror incident.

The administration’s public assessment began to shift when Defense Secretary Chuck Hagel told Congress in a morning appearance that the attacks were "a cruel act of terror."

Appearing on television a short while afterward, Obama said the events in Boston were a "heinous cowardly act, and given what we now know about what took place, the FBI is investigating it as an act of terrorism."

"Any time bombs are used to target innocent civilians it is an act of terror. 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. That’s what we don’t yet know."

The president praised those who had come to the aid of the injured.

"If you want to know who we are, what America is, how we respond to evil, that’s it: selflessly, compassionately, unafraid," he said.

Obama stepped to the microphone after receiving a briefing at the White House from Attorney General Eric Holder, Homeland Security Secretary Janet Napolitano and other top aides.

The bombs exploded on Monday afternoon near the finish line of the famed Boston Marathon, an annual 26 mile race through the neighborhoods of the city.