Following a briefing with intelligence officials, Maryland Rep. C.A. Dutch Ruppersberger, top Democrat on the House Intelligence Committee, said most urban areas in the country would be under high alert.
"We want to make sure this is not a pattern," Ruppersberger said, adding that people could expect to see greater security at public areas such as train stations, ports and baseball games.
On Capitol Hill, Sen. Dianne Feinstein, D-Calif., told reporters that she had been in contact with U.S. intelligence agencies and it was her understanding "that it's a terrorist incident." Feinstein, who chairs the Senate Intelligence Committee, said the officials reported no advance warning that "there was an attack on the way."
California Republican Rep. Ed Royce, chairman of the House Foreign Affairs Committee, said it was a "terrorist attack" and "yet another stark reminder that we must remain vigilant in the face of continuing terrorist threats."