Suspending his special-election campaign activities Democratic Senate candidate Ed Markey said he was "disturbed" by the news.
"I am disturbed and saddened by the explosions at the site of the Boston Marathon this afternoon. We all are grateful for the first responders who rushed to the scene to help the victims," Markey said in a statement released to reporters. "The heart of the city is hanging heavy, and our thoughts and prayers are with everyone affected by this senseless tragedy."
Sen. Dianne Feinstein, who chairs the Senate Intelligence Committee, said she felt there was no doubt the explosions were an act of terrorism.
"It could be foreign, it could be homegrown," Feinstein told ABC News in an interview at the Capitol. Feinstein said the incident in Boston had all the "hallmarks" of a terrorist attack. The senator said she has been briefed by intelligence officials and would receive a formal briefing tonight.
Senators Susan Collins and Angus King of Maine compared the explosion to an act of terrorism.
"As members of the Senate Intelligence Committee, we will be continually updated of the situation. In the meantime, initial press reports that multiple improvised explosive devices may have been involved at this high profile national event bear the hallmarks of a terrorist attack," the senators wrote in a joint statement.
Senator Mary Landrieu of Louisiana, a member of Homeland Security Committee, told ABC News that she and her colleagues on the committee were expecting to receive a briefing tonight on Boston.
She said "it's too soon" to speculate on who is responsible.
Authorities in Boston, Philadelphia, and Washington, D.C., were on high security alert after the explosions.
The District of Columbia's public transit system, Washington Metropolitan Area Transit Authority (WMATA) stepped up security measures spurred by what they called "an abundance of caution."
WMATA encouraged riders on Twitter to call Transit Police (202-962-2121) if they noticed anything suspicious in the system.
A release from WMATA said that Metro Transit Police Chief Ronald Pavlik asked all officers serving during the day stay on-duty through the rush-hour period in the evening.
"We're going to continue to monitor the events in Boston and we'll take appropriate measures," said WMATA public information officer Philip Stewart. "If heightened security is appropriate we will make that measure."