The death of Osama bin Laden left a vacuum not just at the top of Al Qaeda's power structure, but at the top of the Federal Bureau of Investigation's Most Wanted Terrorists list. Today, at least one of those gaps has been filled as Ayman Al Zawahiri moved to the no. 1 position on the FBI's roundup.

A source within the bureau  said the photo of Osama bin Laden with the red "deceased" mark across his face had been left up for three and a half weeks after his death had been there in part to drive home the point that U.S. forces had killed the terrorist leader, and in part to hold the spot open while the FBI determined who would officially fill it. A call for comment to the head of the bureau's press office, Paul Bresson, was not immediately returned today (See update below).

News outlets have been speculating since Bin Laden's death on who would get the dubious honor of being named no. 1 on the main Top 10 list. On Saturday, the Scripps Howard News Service reported, "The bureau has sent out a message to all 56 field offices soliciting nominees" before FBI director Robert S. Mueller, III would make the final decision.

The move suggests that while Saif al-Adel has stepped into a temporary "caretaker" role at the top of the terrorist organization, the FBI is betting on Al Zawahiri to take over permanently. The Egyptian physician has "been indicted for his alleged role in the August 7, 1998, bombings of the United States Embassies in Dar es Salaam, Tanzania, and Nairobi, Kenya," according to the Web site.

An aside to the out-of-work Bin Laden hunters out there: There's a $25 million reward for information "leading directly to the apprehension of conviction" of Al Zawahiri, so get back on the hunt. Seven of the remaining nine most wanted carry rewards of $5 million, one is for $1 million, and one is for $250,000.

Update (May 20, 3:20 p.m.): Bresson has clarified that while Al Zawahiri has moved into Bin Laden's place on the Web representation of the most wanted terrorists list, the Bureau doesn't necessarily rank those suspects the way they do their main 10 Most Wanted list. "We want to find each one of them just as much," he said. But Al Zawahiri carries by far the highest reward, at $25 million. The next-highest reward on the list is $5 million. Bresson said that the rewards are determined by both the FBI and the State Department.