Defense Secretary Chuck Hagel declared Tuesday that the Boston Marathon bombing attack was a "cruel act of terror," and that the Pentagon is prepared to respond in any way that is needed.
Hagel's comments were the strongest statement yet by a member of the Cabinet that the attack -- which killed at least three and wounded more than 170 -- was terrorism.
Hagel said it is "still not determined" whether the plot was foreign or domestic in origin. Officials at all levels of government are working to investigate who is responsible and how broad the plot may have been. President Obama vowed Monday that those behind the deadly attack would "feel the full weight of justice."
Obama did not call the attack "terror" in his brief remarks from the White House briefing room, but White House officials -- and now Hagel -- subsequently did use that label.
"(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.
Hagel spoke about the explosion during testimony before a House Appropriations subcommittee.
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.
"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts," Obama said Monday. "But make no mistake, we will get to the bottom of this. ... We will find out who did this, and we will hold them accountable."
Elsewhere in Washington, officials were honoring the victims with moments of silence, solemn remarks and lowered flags. The White House was joining the U.S. Capitol Monday in lowering its flag to half-staff.
A proclamation from the president said flags at the White House and all public buildings and military posts would be flown at half-staff until sunset April 20 "as a mark of respect for the victims of the senseless acts of violence perpetrated" in Boston.
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."
Fox News' Justin Fishel contributed to this report.