Members of Congress are calling the bombings at the Boston Marathon acts of terrorism and vowing to bring the perpetrators to justice.
Texas Republican Michael McCaul, the chairman of the House Committee on Homeland Security, says the question is whether the terrorism is foreign or domestic.
California Democrat Dianne Feinstein, who chairs the Senate Intelligence Committee, also called it a "terrorist incident."
President Barack Obama didn't use the word in his brief remarks, but a White House official later said the incident was being treated as terrorism.
A U.S. intelligence official says two other bombs were found near the end of the course. There's been no quick claim of responsibility.
Meanwhile, Capitol Hill police say there is no known connection between the bombings against the Boston Marathon and Washington but they are warning Congress and their staffers to be vigilant.
A warning notice sent out to staff, obtained by The Associated Press on Monday, says the Capitol Police are stepping up their numbers in anticipation of more security checks and more suspicious packages reported over the coming days. They also warned of tighter security at entryways, including more frequent canine inspections of visitors.
The notice says the "increased presence and visibility is a proactive response" to the events in Boston, rather than being based on any specific threat.
Many congressmen called the bombings terrorism, but said it was too soon to know if the attack or attackers were domestic or foreign.
A key senator says the unveiling of a sweeping immigration bill set for Tuesday could be delayed because of the tragedy at the Boston marathon.
Republican Sen. John McCain of Arizona says the roll-out could happen Wednesday instead.
The bill would secure the border, remake legal immigration, boost workplace enforcement and put 11 million people here illegally on a path to citizenship.
Immigration activists and business and religious leaders had been invited to an event Tuesday to release the bill.
It would be the culmination of months of closed-door meetings among four Democrats and four Republicans, including McCain, Sen. Marco Rubio of Florida, and Democrat Chuck Schumer of New York.
Their legislation aims to strike a balance between enforcement provisions sought by Republicans, and Democratic priorities, including making citizenship accessible.