WASHINGTON • Fifteen years after the Sept 11 attacks, US anti-terror officials say the country is hardened against such well-developed plots but remains as vulnerable as ever to small and especially home-grown attacks.
Counter-terror operations are under huge pressure to ferret out and disrupt plots by sympathisers of the Islamic State in Iraq and Syria (ISIS) militant group and Al-Qaeda hidden by less centralised networks and new communications technologies, they say.
"Our job is getting harder," said Mr Nick Rasmussen, the powerful director of the National Counter- terrorism Centre, at a stock-taking exercise this week in Washington.
The explosion of ways in which extremists can communicate with one another, many of them via popular smartphone apps and easy access to powerful encryption, "gives them the edge" against the US intelligence community, he said.
The Sept 11 attacks gave birth to the US war on terror, which initially focused on Al-Qaeda and the Taleban. But 15 years later, the target is a different group, ISIS, which has seized territory in Syria and Iraq and shown the ability to plan and inspire home-grown attacks in Europe and the United States, smaller- scale than Sept 11 but nevertheless deadly and demoralising.
Meanwhile, Al-Qaeda still exists without former leader Osama bin Laden, with affiliates, spin-offs and rivals of both groups operating from the Philippines to West Africa, posing a more complex threat.
"The reality is that it has metastasised" from the Iraq-Syria region, said Dr Frank Cilluffo, director of the Centre for Cyber and Homeland Security at George Washington University. "The threat persists and is in some cases more complex."
A series of surprise attacks have placed "HVEs" - home-grown violent extremists - as much in the focus of intelligence agencies as threats from abroad. Among them, a 29-year-old American of Afghan descent believed to hold radical Islamic sympathies shot dead 49 people at a gay nightclub in Orlando, Florida, in June. And last December, a US-born man and his wife, both with Pakistani roots, killed 14 people at a Christmas party in San Bernardino, California.
The George Washington University Programme on Extremism counts 102 people charged in the US with offences related to ISIS, many of them lured online.
US intelligence is strained by the more than 1,000 cases of possible extremists it is following, Mr Rasmussen said. Moreover, plots are now developed and carried out much more rapidly, and in smaller networks, making it much harder for counter-terror operations to discover them.
Officials say they are confident ISIS will be defeated on its Iraq-Syria turf eventually, but that will not end the overall extremist threat. A break-up of ISIS could send hundreds of sympathisers underground around the world, lying quietly in wait for years to build new networks and plot attacks, they say.
The other big challenge, the officials say, is the weakness of European intelligence when it comes to identifying and tracking threats, which they link to still-weak cooperation between agencies in the different countries.
The core fight is in ideology, officials also say. But the US has so far made little progress in combating the propaganda that draws sympathisers to ISIS and Al-Qaeda.