Al Qaeda's core leadership is badly wounded and almost certainly incapable of mounting another attack like the one on September 11, 2001, in New York and Washington, according to US and European security officials.
But even as the threat of spectacular, coordinated mass-casualty attacks by al Qaeda seems to have faded, it has been replaced by new worries -- the network's violent spinoff groups and individual radical "lone wolves," to name two.
In an illustration of such concerns, US officials said on Thursday there was a credible but unconfirmed threat involving Washington and New York ahead of Sunday's 10th anniversary of the attacks on those cities.
Official have said that intelligence gathered from the raid that killed Osama bin Laden last May highlighted the al Qaeda leader's persistent interest in attacking the United States around the anniversary of the 2001 attacks. But it is unclear if those plans ever evolved beyond aspiration.
"AQ Central has never been weaker, they have been pounded into submission" by CIA drone attacks, said Roger Cressey, a former top White House counterterrorism official, referring to al Qaeda by its initials.
"If the threat was prioritized as AQ Central, the affiliates and self-radicalized individuals in that order after 9/11, the opposite order is true today," Cressey said.
The near-demise of al Qaeda, the Islamic militant network that grew out of the fight by bin Laden and fellow Arabs to expel Soviet troops from Afghanistan in the 1980s, goes beyond bin Laden's killing by US forces in Pakistan.
The latest milestone was the killing last month in a US drone strike in Pakistan of Atiyah abd al-Rahman, a Libyan whom US officials called the No. 2 to Egyptian Ayman al-Zawahri, bin Laden's successor as al Qaeda chief.
Rahman was the latest target of the dramatically intensified US drone campaign which, for all the controversy it has sparked in Pakistan and elsewhere, has become a lethal weapon for which al Qaeda leaders have offered no adequate answer.
A senior US official, speaking on condition of anonymity, said part of the significance of Rahman's demise was that unlike bin Laden, he tried to operate below the radar of Western spy agencies. Yet he was still identified, located and killed.
The vacuum created by the disintegration of al Qaeda's central command is being filled by Qaeda "franchises" -- spinoff or copycat branches of bin Laden's original network, counterterrorism officials say.
"The movement fueled by a common ideology has morphed into more of an AQ hydra, with the old core weakened but new franchises and inspired individuals taking on the global jihadi mantle," said Juan Zarate, a White House counterterrorism adviser to former President George W. Bush, referring to the multi-headed serpent of Greek mythology.
Al Qaeda propagandists and apologists have also established a formidable presence on the Internet to promote the group's ideology and indoctrinate militant wannabes.
A worrisome development is the proliferation of individual violent militants -- the "lone wolves" -- who operate unseen by intelligence agencies and police and can create mayhem with a carful of home-made explosives or guns. The result is a lower risk of future large conflagrations but a growing threat of smaller attacks that could be harder to detect and thwart.
"Future attacks against America will be less complex, less well organized, less likely to succeed, less lethal if they do succeed. They will just be more numerous," said retired General Michael Hayden, who led the CIA and National Security Agency.
Bruce Riedel, a former senior CIA analyst who has advised Obama on counterterrorism policy, sounded a note of caution about the original al Qaeda. "Al Qaeda's old core is badly wounded but still has powerful allies like the Pakistani Taliban that can serve as force multipliers," Riedel said.
Riedel said the next iteration of al Qaeda may be a proliferation of militants "trained for one-time missions to hemorrhage the US" -- people like Faisal Shahzad.
The Pakistan-born US citizen radicalized himself through the Internet, spent a few days with militants in Pakistani tribal areas, then tried last year to attack New York's crowded Times Square with an incompetently built car bomb.
Al Qaeda's core group headed by Zawahri retains a training and propaganda capability, US and European officials say.
Its resources for training field operatives are nothing like the system of relatively sophisticated paramilitary encampments it operated in Afghanistan before the 2001 attacks. At best, officials said, al Qaeda's central command can organize small-scale, temporary and discreet training sessions in remaining safe-havens in Pakistan's tribal areas.
The results of such efforts are mixed. Shahzad trained for a week or two with suspected al Qaeda militants in North Waziristan. But he failed to build a gasoline-based bomb that could actually explode. He was arrested at New York's JFK Airport as he tried to flee the United States.
Al Qaeda affiliates in Yemen (known as Al Qaeda in the Arabian Peninsula, or AQAP) and north Africa (known as Al Qaeda in the Islamic Maghreb) are seen as the best-organized franchises of bin Laden's original network.
Somalia-based al Shabaab, which has recruited native Somalis in the United States and has growing ties to the Yemen affiliate, is also seen as a major concern.
The Yemen-based group is viewed with particular wariness because it has shown the capability for imaginative attack tactics such as underwear and printer-cartridge bombs. It also has been working, intelligence reports say, on a grisly innovation: bombs that would be surgically implanted inside a militants' body to deceive security screeners.
Its ambitions sparked particular concern in the US government because of the role in it played by Anwar al-Awlaki, an American-born imam who US officials believe has built a substantial following in the United States and other Western nations through English-language postings on the Internet.
A senior US official, speaking on condition of anonymity, said Awlaki "pulled back" from public activities in recent months amid growing interest in him by US and European intelligence agencies. Awlaki and others in the group have "isolated themselves" from the Internet and other electronic devices to improve their security, the official said.
One of the biggest concerns about Awlaki is his success in attracting and inspiring disaffected young Muslims, some of them converts to Islam.
Major Nidal Malik Hasan, the US Army psychiatrist who is charged with killing 12 people in a shooting spree at the Fort Hood military base in Texas in 2009, was an Awlaki admirer and e-mailed the preacher. The extent to which Awlaki responded is unclear.
Investigators said "lone wolves" sometimes become radicals on their own without direct contact with other militants. These people, who get their ideology and potential tactical guidance exclusively from their computer screen, are difficult, if not impossible, for intelligence and security agencies to detect.
An August report by the RAND Corp. found that al Qaeda's use of the Internet to recruit home-grown US militants had largely failed, with only 10 of 32 plots going beyond the discussion stage, and six of those 10 broken up by FBI stings.
"America's home-grown jihadist terrorists have not shown great determination or very much competence," said Brian Michael Jenkins, the study's author.