(USA TODAY) -- President Obama said Tuesday the FBI is investigating the Boston Marathon bombings as an "act of terrorism," but the perpetrators and motives remain a mystery.

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

The president said, "clearly, we're at the beginning of our investigation," but he said investigators will learn what happened and why: "We will find whoever harmed our citizens, and we will bring them to justice."

Obama, who had held off using the term "terrorism" in brief remarks on Monday night, spoke again after being briefed by FBI Director Robert Mueller and other officials on Tuesday morning about what he called the "heinous and cowardly" crime.

"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."

In addition to Mueller, the president was briefed by Attorney General Eric Holder, Homeland Security Secretary Janet Napolitano, and White House counter-terrorism adviser Lisa Monaco.

Regardless of who carried out the attack, Obama said "the American people refuse to be terrorized."

The president spoke of the "heroism and kindness and generosity and love" of those who rushed to help in the aftermath of the twin bombings near the finish line of Boston Marathon. He cited "exhausted runners who kept running to the nearest hospital to give blood and those who stayed to tend to the wounded, some tearing off their own clothes to make tourniquets."

Added Obama: "So if you want to know who we are, what America is, how we respond to evil, that's it. Selflessly, compassionately, unafraid."

While vowing to bring the perpetrators to justice, Obama said authorities don't know whether it was carried out by a group or an individual or individuals, or what the motive might have been.

"So everything else at this point is speculation," he said.

The bombing has killed at least three people and injured more than 170 others.

Shortly before his remarks, Obama ordered flags at the White House and other government buildings be flown at half-staff in honor of the victims of Monday's attack.

The flag lowering is "a mark of respect for the victims of the senseless acts of violence perpetrated on April 15, 2013," Obama said in a proclamation.