(CNN) -- Secretary of Defense Leon Panetta says there is no question that the United States is safer with Osama bin Laden, the architect of al Qaeda, dead, but contends there is no "silver bullet" to destroy the terror network.
Panetta's assessment came just days before the one-year anniversary of the U.S. commando raid on bin Laden's compound in Abbottabad, Pakistan, that ended a manhunt for the al Qaeda founder that began following the September 11, 2001, terror attacks.
"Having been involved in operations even before bin Laden, it was clear that there is no kind of silver bullet here to suddenly be able to destroy al Qaeda and that includes going after bin Laden," Panetta told reporters Friday after a meeting with defense counterparts in South America.
"The way this works is that the more successful we are in taking down those that represent their spiritual and ideological leadership, the greater our ability to weaken their threat to this country."
The FBI and the Department of Homeland Security last week warned of the possibility of terrorist attacks leading up to and after the anniversary of the May 2, 2011, killing of bin Laden. There is no specific, credible terror threat, the agencies said.
The warning, released by the FBI and DHS, says individuals have posted messages on "violent extremist Web forums" vowing attacks on the United States around the anniversary, but adds that "such threats are almost certainly aspirational."
"I don't think there's any question that America is safer as a result of the bin Laden operation," Panetta said."... It doesn't mean that they don't remain a threat. It doesn't mean that we somehow don't have the responsibility to keep going after them wherever they are."
He described the raid that he oversaw as the then-CIA director as "tense," with several "nerve-wracking moments."
Panetta recalled the moment during the raid when one of two helicopters used by Navy SEALs crashed at bin Laden's compound in Abbottabad, Pakistan.
"Just the fact that having those helicopters going 150 miles into Pakistan and the concern about whether or not they would be detected. And then actually going in to the compound when one of the helicopters went down because of the heat coming off the ground. It was just hotter than anybody had anticipated. And obviously that was pretty nerve-wracking," he said.
"... Fortunately, we had a back-up helicopter that came in and was able to pick up the people that were there."
The biggest question during the operation, Panetta said, was "whether or not bin Laden was really there."
"We had no specific information that he was actually located there. All we had was, you know, just a lot of circumstantial intelligence and information. But all of us were, were kind of holding our breath to find out whether or not he was actually there."
The answer came about 20 minutes after the SEALs entered the bin Laden compound, with the utterance of the code word "Geronimo."
"There was a huge sigh of relief by everybody involved in that," he said.
While many details of the raid are known, questions remain about how bin Laden could have been in living for years in a compound that was only a mile from the Pakistan Military Academy in Abbottabad, a largely military community outside the capital, Islamabad.
The U.S. raid, which was conducted without the knowledge of Pakistan, enraged the Pakistani public and deploy embarrassed its military.
Panetta has said he remains convinced that someone with authority in Pakistan knew bin Laden was hiding at the compound. That claim has been denied by Pakistani officials.
U.S.-Pakistani relations remain tense, in part because of U.S. drone strikes inside the country. Pakistan has in essence halted much of its cooperation with the United States while its parliament reassess future terms of engagement with Washington following a U.S. airstrike in November that killed 24 Pakistani soldiers along the border with Afghanistan.