Bin Laden's deputy, Ayman al-Zawahiri, is expected in some circles to take over, but it is not clear how soon that will happen and he may be more concerned for his own public safety after US special operations took out the Al-Qaeda chief in a daring raid.
"He's moving up very fast on the list," Panetta told CBS television's "Evening News with Katie Couric" in referring to Zawahiri.
Panetta said that until a new Al-Queda leader is formally anointed, the Untied States expects to take advantage of the situation.
"We think that'll give us some opportunities to be able to continue to attack them in the confusion and debate that they're going to go through as to who ultimately replaces bin Laden," he said.
"But I can assure you, whoever takes his place, he will be number one on our list."
Bin Laden's hideout in a large compound in the garrison town of Abbottabad, just north of Islamabad, has led to widespread speculation that the former Al-Qaeda leader's whereabouts had been known to the Pakistani government.
But Panetta said the US government does not have any intelligence indicating that "Pakistan was... aware that bin Laden was there, or that this compound was a place where he was hiding."
Noting that the compound was close to an elite military academy and that bin Laden had been living there about five years, Panetta nonetheless pressed Islamabad to provide more answers.
"I just think they need to respond to the questions about why they did not know that that kind of compound existed," he said.
Asked whether Pakistan should be declared a terrorist state, Panetta acknowledged that the US-Pakistani relationship is a "very complicated and difficult" one, but warned the ties should not be severed.
"Look, we are virtually conducting a war in their country going after Al-Qaeda," he said. "And at the same time, we're trying to get their help in trying to be able to confront terrorism in that part of the world.
"And they have given us some help, and they have given us some cooperation."
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