The United States and its allies braced Monday for possible retaliation from al Qaeda after U.S. forces killed its leader, Osama bin Laden, in Pakistan and his supporters vowed revenge.
Counterterrorism experts said the killing might herald a shift in the balance of power among the affiliates that make up the al Qaeda network, with Al Qaeda in the Arabian Peninsula (AQAP), the Yemen-based offshoot that U.S. officials believe is the most dangerous, assuming more of a leadership role.
"Though bin Laden is dead, al Qaeda is not," CIA Director Leon Panetta said in a statement. "The terrorists almost certainly will attempt to avenge him, and we must - and will - remain vigilant and resolute."
Bin Laden for many years had little day-to-day involvement in al Qaeda, and his death likely will be a symbolic blow to the organization, albeit a destructive one, as unflattering details emerge about his life in hiding and if uncertainty grows about who will succeed him.
The men with a more operational role in al Qaeda Central in Pakistan and Afghanistan, Abu Yahya al-Libi and Ayman al-Zawahiri - the latter widely believed to be bin Laden's designated successor - will be under "enormous pressure" as U.S. forces seek to leverage the intelligence they found in the raid and follow up on their success, former State Department counterterrorism official William McCants told The Washington Times.
The additional pressure might make it hard for Mr. al-Zawahiri to publicly respond to the killing, leaving a vacuum, Mr. McCants said.
"AQAP looks much better positioned to step up and fill that vacuum - providing operational leadership and strategic guidance" for the decapitated global terror movement, he said.
In Yemen, a member of AQAP called bin Laden's death a "catastrophe," and other scholars suggested there is no real replacement for him.
"There's no substitute for Osama," Peter Bergen of the New America Foundation said.
Even Anwar al-Awlaki, an English-speaking cleric who has become the Internet-friendly face of Islamic extremism, "doesn't have half the stature bin Laden did," Mr. Bergen said.
"He's never fought in a war. Next to Osama, he's a dwarf. ... They all are," he said of other extremist leaders.
Both Mr. Bergen and Mr. McCants agreed that Mr. al-Zawahiri likely would be a poor leader for al Qaeda. "He is a terrible manager, not well-regarded or well-liked even by his own followers," Mr. Bergen said.
Both scholars said they expect that some kind of video statement, specially recorded by bin Laden for use in the event of his death, would be released soon.
A spokesman for the Pakistani Taliban, an extremist group closely linked to al Qaeda, vowed revenge on U.S. and Pakistani forces. "If he has been martyred, we will avenge his death," Ehsanullah Ehsan told reporters.
But there appeared to be some confusion among the group's members, and another spokesman claimed that bin Laden was alive and well, according to Raheel Khan, bureau chief for Radio Free Europe's Pashto-language service, Radio Mashaal.
"Only the mid-level cadres have been speaking," he told The Washington Times.
On Internet forums for al Qaeda supporters, some welcomed the news that bin Laden had achieved martyrdom, while others said they were praying it was not true.
"If it is true, then we must thank Allah that America was not able to capture him alive," wrote one poster, noting the "humiliating" video of Saddam Hussein being given a medical examination that the U.S. military released after his capture.
Other contributors were skeptical about the U.S. account, urging their fellows not to believe bin Laden was dead until the news was confirmed by official al Qaeda sources.
"The source of news that we trust is that which comes from the mujahideen," one counseled. "Be patient and don't spread rumors."
Hours later, several of the threads discussing bin Laden's killing were shut down and the posts removed.
Mr. Khan said he expects a violent response from bin Laden supporters when the news sinks in. "We may see an increase in terrorism," he said. "Thousands of people across the world still believe" in bin Laden's ideology.
Officials in Washington said they had no specific intelligence about a possible retaliatory attack. "There may be a heightened threat to the homeland and to U.S. citizens and facilities abroad," a senior administration official told reporters at the White House.
The Department of Homeland Security said it was not raising the national threat level, but the agency sent an intelligence bulletin to law enforcement warning of the possibility of spontaneous lone-wolf attacks by al Qaeda supporters.
"I have been clear ... that we will only issue alerts when we have specific or credible information to convey to the American public," Homeland Security Secretary Janet Napolitano said. "We remain at a heightened state of vigilance."
The State Department said it had sent guidance to embassies worldwide and issued a travel advisory for Pakistan.
In Washington, Secretary of State Hillary Rodham Clinton sought to use news of bin Laden's death as a lure to get the Taliban to join the U.S. in peace talks in Afghanistan.
"Our message to the Taliban remains the same," she said. "But today, it may have even greater resonance: You cannot wait us out. You cannot defeat us. But you can make the choice to abandon al Qaeda and participate in a peaceful political process."
c This article is based in part on wire service reports.
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