Obama: Boston attack a 'heinous' act of 'terrorism'

President Obama said Tuesday that the "heinous" attack on the Boston Marathon is being treated as an "act of terrorism," while investigators try to determine whether the bombing was the work of a terrorist group or "malevolent individual."

"This was a heinous and cowardly act, and given what we now know about what took place, the FBI is investigating it as an act of terrorism," Obama said.

"Any time bombs are used to target innocent civilians, it is an act of terror."

The president spoke in more depth about the attack after delivering a brief statement from the White House Monday evening.

At the time, he did not refer to the strike as terrorism, though White House officials and Defense Secretary Chuck Hagel later did. Hagel on Monday called the explosion a "cruel act of terror."

The president stressed Tuesday that officials still do not know who carried out the attack or why -- or whether it was planned by a domestic or foreign terror group, or a lone-wolf individual attacker. He described the investigation as being in the early stages.

"We will find whoever harmed our citizens, and we will bring them to justice," he said, while urging Americans to remain vigilant and report suspicious activity.

Officials say at least three were killed and more than 170 wounded in the pair of explosions that ripped through the finish line at the Boston Marathon Monday.

Shortly before speaking Tuesday, the president also issued a proclamation for flags at the White House and all public buildings and military posts to be flown at half-staff until sunset April 20. The proclamation said Obama ordered the lowering "as a mark of respect for the victims of the senseless acts of violence perpetrated" in Boston.

Earlier, Hagel delivered a strong condemnation of the attack during testimony before a House Appropriations subcommittee.

"(The attack) is clearly an act of terror and will be approached as an act of terror," Hagel said.

He said the National Guard were among the first on the scene. He said the Defense Department is prepared to respond quickly to any request from domestic law enforcement.

While counterterrorism officials pore over data in an effort to search for advance threats and clues, the president is being regularly briefed by his Homeland Security team.

On the floor Tuesday, House Speaker John Boehner said the attack is "a reminder of just how vulnerable we really are in this era of modern warfare."