A White House official, speaking on condition of anonymity because the investigation was still unfolding, said Monday's Boston Marathon bombings are being treated as an act of terrorism.
President Obama did not use the word terrorism in his first comments on the bombings, but he vowed that those responsible will "feel the full weight of justice."
"We still don't know who did this or why," the president said in a nationally televised statement. "Make no mistake: we will get to the bottom of this."
The Secret Service shut down Pennsylvania Avenue outside the White House as a security measure shortly after the bombs exploded in Boston.
Apparently, the nation's capital will proceed with an Emancipation Day parade on Tuesday morning.
Authorities shed no light on a motive or who may have carried out the Boston bombings, and police said they had no suspects in custody.
But The New York Post reports that a 20-year-old Saudi national is being held under guard in an undisclosed Boston hospital, according to law enforcement sources.
A senior intelligence official divulged that in addition to the two bombs that exploded in the packed streets of Boston, two other bombs were found near the end of the 26.2 mile course in what appeared to be a well-coordinated attack.
With scant information to guide them, members of Congress suggested there was little or no doubt it was an act of terrorism.
"We just don't know whether it's foreign or domestic," said Rep. Michael McCaul, R-Texas, chairman of the House Committee on Homeland Security.