In Zawahiri's most recent video, last month, he called for his followers to focus on the U.S.
"Hunt her down wherever you may encounter her. Hunt her down to cut what is left of her corruption's tail," Zawahiri says in the video.
"We know from Bin Laden's own handwriting he wanted to do an attack around the anniversary," said Clarke. "We know from his successor's own audio tapes and video tapes that he feels he has to prove al Qaeda is still alive by avenging bin Laden's death. And we know that this kind of technique could be relatively easily done, even by an al Qaeda that's on the ropes, even by an al Qaeda that has very few people left."
Aaron Katersky and Lee Ferran contributed to this report.