With Abu Musab al-Zarqawi dead, will Osama bin Laden and al-Qaida launch an attack to prove they are still a serious threat?
"Zarqawi was their principal partner, and with him out of the picture al-Qaida will probably look to do something to show that they are still relevant," says Michael Sheehan, an NBC News terrorism analyst.
U.S. intelligence agencies have warned of possible retaliation.
"I am confident that al-Qaida will try to regroup and kill other people in order to say, 'Well, we haven't lost our, you know, we haven't lost our way,'" said President Bush on Friday.
But in recent years, bin Laden and Ayman al-Zawahri, his top deputy, have been more talk than action. They release propaganda tapes, like one that aired on al-Jazeera Friday praising al-Zarqawi, without mentioning his death. The CIA says it was recorded recently but before Wednesday's attack.
So how much of a threat is al-Qaida?
Its last major bombing was two years ago, on July 31, 2004, in Tashkent, Uzbekistan.
But they have inspired splinter groups and imitators — with devastating effects, including the July 2005 bombings in London and the March 2004 attacks in Madrid.
Intelligence czar John Negroponte on Friday predicted an attack outside the U.S.
"Such areas as Iraq, Afghanistan and perhaps other locations in the world," he said.
Targets could include Jordan, where al-Zarqawi had an organization, or events like the World Cup soccer finals in Germany.
"The timing is going to be something that is dependent on how far advanced their operational activities are, and how capable they are at carrying out a successful attack," says John Brennan, a former counterterrorism official.
In fact, al-Qaida's imitators have proved less capable than bin Laden of launching major attacks like 9/11. But the U.S. remains vigilant, to see how al-Qaida responds to the death of its leader in Iraq.