President Barack Obama, speaking from the White House late Monday, pointedly avoided using the words “terror” or “terrorism,” saying officials “still do not know who did this or why.” However, a White House official later said the incident at the famous race was being treated as terrorism.

Following a briefing with intelligence officials, Maryland Rep. C.A. Dutch Ruppersberger, the top Democrat on the House Intelligence Committee, said most urban areas in the country would be under high alert.

As many as two unexploded bombs were also found near the end of the 26.2-mile course as part of what appeared to be a well-coordinated attack, but they were safely disarmed.

As the FBI took charge of the investigation, authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. Officials in Washington said there was no immediate claim of responsibility.

WBZ-TV reported late Monday that law enforcement officers were searching an apartment in the Boston suburb of Revere. Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night in Revere but provided no further details.