“As this situation continues to develop and our first responders continue their effort to ensure the safety of our friends and family in Boston, there are still far more questions than answers,” said Sen. Mo Cowan (D-Mass.), who was appointed to John Kerry’s seat after the longtime senator became secretary of State.

Senate Homeland Security and Governmental Affairs Committee Chairman Tom Carper (D-Del.) said members were carefully monitoring the incident as it unfolded.

“In the end, we’ll get to the bottom of this incident and use any and all information from this horrible tragedy to bolster our security and response efforts and continue to do all that we can to keep events like this from happening again,” he said.

And that turned to franker talk of terrorism among many lawmakers.

“Our first responsibility is to secure the support the victims and their families will need in the days and months ahead,” said Sen. Jerry Moran (R-Kansas). “We must identify those who committed this cowardly act, as well as those who encourage them through actions or silence, and hold them fully accountable for their crimes.”

“Though it remains unclear who is responsible, this attack is yet another stark reminder that we must remain vigilant in the face of continuing terrorist threats,” said House Foreign Affairs Committee Chairman Ed Royce (R-Calif.).

“While it’s hard for us to imagine the kind of callousness necessary to so recklessly maim and kill innocents at such a wholesome event, we will not rest until the person or persons responsible for this tragedy are brought to justice,” Sen. John Hoeven (R-N.D.) said.

Sen. Dianne Feinstein (D-Calif.), chairwoman of the Senate Intelligence Committee, told ABC News that the bombings had the “hallmarks” of terror attack. “It could be foreign, it could be homegrown,” she added.

Senate Foreign Relations Committee Chairman Robert Menendez (D-N.J.) called on “my fellow members of Congress to use this attack on American soil to re-commit our commitment to protecting Americans at home and abroad.”

“I am disturbed and saddened that anyone would ruin this day of celebration by targeting innocent people and murdering them in cold blood,” Sen. Marco Rubio (R-Fla.) said. “Once it is determined who is responsible, justice must be served to the fullest extent possible.”

Rep. Sheila Jackson Lee (D-Texas), a member of the House Homeland Security Committee, said she’s “sure” congressional hearings will be called “when the investigations are complete.”

“Within the next few months we must reevaluate the status of the safety and security of America’s large venues and large events that are coming up in the pending weeks and months,” she said.

Homeland Security Chairman Mike McCaul (R-Texas) said he was in contact with Boston authorities and federal officials to try to get information about who’s responsible as it becomes available.

“I would urge the American people to remain calm but vigilant,” McCaul said. “Today’s events are a stark reminder that our enemies continue to plot against us.”