President George W. Bush's administration has crippled al Qaeda's ability to carry out major attacks on US soil but at a political and economic cost that could leave the country more vulnerable in years to come, experts say.
Even as al Qaeda tries to rebuild operations in Pakistan, experts including current and former intelligence officials believe the group would have a hard time staging another September 11 because of US success at killing or capturing senior members whose skills and experience have not been replaced.
"If the question is why al Qaeda hasn't carried out another 9/11 attack, the answer I think is that if they could have, they would have," said a former senior US intelligence official who spoke on condition of anonymity.
Tighter US airport security, greater scrutiny of people entering the United States and better coordination between the CIA, FBI and Department of Homeland Security also have made it harder for extremists to enter the country, experts said. Home-grown extremists in the United States are believed to be isolated and lacking the will or ability to carry out large-scale operations.
"Make no mistake about it, however, our enemy is resilient and determined to strike us again," said Charles Allen, chief intelligence officer at the Department of Homeland Security. Some experts warn that the successes of Bush's war on terrorism have been undercut by huge security costs, strains on the US military from wars in Iraq and Afghanistan and resentment of the United States abroad.
"Look at al Qaeda's plans," said Michael Scheuer, who once led the CIA team devoted to finding al Qaeda leader Osama bin Laden. "They're very simply defined in two phrases: spread out America's forces and bleed the United States to bankruptcy. I'd argue America has been under attack successfully every day since 9/11 from that perspective.
"If you're looking at it from the cave, or wherever al Qaeda is hiding at the moment, you have to be pretty happy with the way the world is moving," he said. The Iraq war has been described by US intelligence as both a cause celebre for new al Qaeda recruits and a militant training ground in explosives and urban guerrilla tactics.
"There may be individuals they've been able to recruit in Iraq who might have the credentials and capabilities to deploy elsewhere, even though the core al Qaeda has been damaged," said John Brennan, former acting director of the National Counterterrorism Center.
US intelligence believes that bin Laden and his second-in-command Ayman al-Zawahri, driven from Afghanistan when US-led forces ended Taliban rule there in 2001, are now trying to re-establish operations in remote, semi-autonomous tribal areas in Pakistan.
But experts view recent attacks in Europe such as the July 2005 London transport bombings as evidence that al Qaeda-linked groups, while dangerous, lack the advanced skills and organisation of militant groups like Hezbollah.
"What al Qaeda's left with is a bunch of Sunni radicals in various capitals who get their orders and technology on the Internet. But their contact with home base is not very strong and they're not very disciplined," said former CIA official Robert Baer. Islamist groups have killed about 1,600 people in 53 attacks overseas since 2001, according to IntelCenter, an Alexandria, Virginia-based intelligence contractor.
The number and lethality of the attacks have fallen off since 2004. Last year, there were five attacks and 28 deaths, according to IntelCenter statistics, which do not include attacks in Iraq, Afghanistan or other war zones.
But IntelCenter chief executive Ben Venzke said the chance of an al Qaeda attack on US soil has grown based on the militant network's increasing references to the American homeland in public messages. "Our leading thinking is that we are closer now to an attempt at a major attack in the United States than at any point since 9/11," Venzke said.