Testifying on Capitol Hill, Hagel was the first Obama administration official to refer to terror or terrorism after the bombings Monday killed three and wounded more than 140 people Monday afternoon.
President Barack Obama, in his own brief statement at the White House late Monday, made no mention of terrorists or terrorism as a possible cause of the bombings. A White House official speaking on condition of anonymity because the investigation was still unfolding did say the attack was being treated as an act of terrorism.
Hagel said any event in which explosive devices are used is clearly an act of terror.
"As the president said yesterday, we still do not know who did this and why, and a thorough investigation will have to determine whether it was planned or carried out by a terror group, foreign or domestic," Hagel told the House Appropriations defense subcommittee.
He mentioned the Pentagon's connection to the race, with many in the defense community participating in the race and commended the quick work of the Massachusetts National Guard to assist after the explosions.
Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, also praised the National Guard. Dempsey was testifying with Hagel.
Hagel said the thoughts and prayers of those at the Pentagon are with the people of Boston.
House Speaker John Boehner, R-Ohio, also referenced terrorism.
"You can describe it in a lot of different ways, but it was a terrorist attack of some sort. ... There's just not enough information at this time," Boehner told reporters Tuesday at a news conference.
"We just don't know enough about it, but I have no doubt that we will. ... The president and I had this conversation last evening. He'd like to know more, I'd like to know more. The American people would like to know more. Unfortunately we don't, but I am confident that we are going to get to the bottom of this."
At the House hearing, Rep. C.W. Bill Young, R-Fla., pressed Hagel about Pentagon plans to disband one of the civil support teams that responded to the attack in Boston. Young said Congress was notified in a March 29 letter that the two teams, one based in New York, the other in Florida, would be disestablished.
Hagel insisted that the teams would remain and said the budget proposal that the administration submitted last week included money for the teams.
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Associated Press writer Jim Abrams contributed to this report.
