The killing of Osama bin Laden marks the culmination of a counterterrorism campaign that made decapitating the al-Qaeda network its paramount goal.

But al-Qaeda has metastasized in the decade since the Sept. 11, 2001, attacks, expanding its reach and adapting its tactics in ways that make the organization likely to remain the most significant security threat to the United States despite its leader’s demise.

In recent months, the nation’s top intelligence officials have testified that al-Qaeda’s affiliate in Yemen probably poses the most immediate threat to U.S. interests, and has been tied to a series of near-miss attacks, including the attempted bombing of a Detroit-bound plane on Christmas Day 2009.

One of the leaders of al-Qaeda in the Arabian Peninsula, as the Yemen-based offshoot is known, is an American-born cleric, Anwar al-Aulaqi, who might stake claim to at least part of bin Laden’s mantle as a charismatic figure committed to attacks against the United States and the West.

Al-Qaeda has established ties to militant movements in other countries, such as Somalia. And even in Pakistan, where bin Laden was killed, the terrorist group functions in some ways as a force multiplier for other militant organizations, including the Afghanistan Taliban — lending expertise and inspiration to groups increasingly capable of wreaking devastation on their own.

President Obama acknowledged in his late-night speech Sunday that bin Laden’s death did not mark the end of the al-Qaeda threat. “There is no doubt al-Qaeda will continue to pursue attacks against us,” Obama said. “We must, and we will remain, vigilant at home and abroad.”

Counterterrorism officials and experts described bin Laden’s death as a devastating blow to the terrorist network he helped found and build. For nearly a decade after the Sept. 11 attacks, bin Laden loomed as a symbol to emerging Islamist extremists, a man who had bloodied the most powerful nation in the world and seemed until Sunday as if he might elude that country’s desperate grasp.

But experts also said that bin Laden in recent years had served much more as a symbol than as an operational figure, and that the implications of his death for al-Qaeda are not entirely clear.

“Decapitating the movement will not undermine it; the al-Qaeda affiliates and the singletons will still pose threats,” said John McLaughlin, a CIA veteran who served as interim director of the agency. “But much of the inspirational power of the al-Qaeda center will diminish.”

Another retired CIA veteran, Paul Pillar, who served as the agency’s national intelligence officer for the Middle East and South Asia from 2000 to 2004, said al-Qaeda has been widely decentralized, spreading to Yemen-based al-Qaeda in the Arabian Peninsula and other organizations, and that bin Laden’s death would have little impact on the planning of attacks.

“In terms of operational control and direction, most of the change that matters has already taken place,” he said. Bin Laden’s role “for some time has been more as a symbol and a source of ideology than an instigator of operations. That role will continue, dead as well as alive.”

Pillar said bin Laden’s demise would “have far more significance in the way that we in the United States and the West react to it than how violent Islamists will be going about their business.”

Most al-Qaeda-related activity in recent years “has been initiated away from the center, on the periphery,” Pillar said. “To the extent there have been more meaningful operational links in recent years, it has been al-Qaeda in Yemen more than anyone else.”

Like other analysts, Pillar said he anticipated an upsurge of terrorist violence in the short term. “I’d attribute that now to an extra incentive to show that the movement is still alive and kicking and the death of this most prominent figure has not meant the death of al-Qaeda.”

As a result, the United States is likely to continue for some time to employ the same tools — drone strikes and multilayered intelligence-gathering efforts — that have been used to increase pressure on al-Qaeda in Pakistan and played a critical role in locating bin Laden more than a decade after he became the main focus of the CIA hunt.

U.S. intelligence officials think that al-Qaeda’s core leadership in Pakistan had been crippled by a relentless campaign of drone strikes over the past three years.

After watching al-Qaeda rebuild its infrastructure in the tribal region of northwest Pakistan, including new training camps and increased recruiting, the Bush administration authorized a dramatic increase in drone strikes against al-Qaeda and Taliban targets.

The use of drone attacks increased still further under Obama. The number of drone strikes in Pakistan increased from five in 2007 to 35 the next year. In 2010, nearly 120 strikes were reported, according to a tally by the Long War Journal Web site.

Director of National Intelligence James R. Clapper Jr., in an assessment of global threats against the United States in February, said the campaign had “greatly weakened al-Qaeda’s core capabilities, including operations training and propaganda.” U.S. officials concluded that, while the group remained capable of carrying out attacks, it was probably unable to organize a Sept. 11-style strike on the West.

But Clapper said al-Qaeda chapters elsewhere — particularly in Yemen and North Africa — continued to pose a serious threat.

“We’re especially focused on al-Qaeda's resolve to recruit Americans and to spawn affiliate groups, most notably its chapter in the Arabian Peninsula,” Clapper said.