On Tuesday, White House press secretary Jay Carney warned of "the evolving threat posed by terrorists."

"There are threats from a variety of al Qaeda offshoots around the world. And, as the president has said and others have said, there is always the potential threat from lone actors — those who are self-radicalized and who are not associated with outside groups or terrorist organizations, but take action on their own," Carney said.

Pew found that the Boston bombings story had some of the highest salience with the news-consuming public of any story since 9/11. Only that terrorist attack, Hurricane Katrina, the 2008 financial crisis, and the D.C. sniper had higher percentages of Americans say they were monitoring the situation "very closely."