Anniversary bomb alert – is al-Qaeda really up to it?

What's happened?Americans have received an alert from the Department of Homeland Security of a "specific, credible, but unconfirmed" threat of a terrorist attack timed for this weekend's tenth anniversary of 9/11. There are very few details, but the target is thought to be either Washington or New York and to involve a truck bomb.

As a result, security has been increased in New York City. Bridges and tunnels into Manhattan will get extra surveillance while New Yorkers will have to endure spot checks of vehicles entering the city and more bag searches on the subway. President Obama, informed of the threat on Thursday morning while he was preparing for his job creation speech to Congress, urged counter-terrorism officials to "redouble" their efforts.

What does it mean?Based on the recent thinking of intelligence experts, the threat - if it is real – is as likely to come from an individual jihadist as from al-Qaeda, which has been shorn of its leader, Osama bin Laden, and at least one senior lieutenant this summer.

An anniversary attack has always been a possibility. Anxiety increased with the discovery of a notebook in the Pakistan hideout of Bin Laden when he was killed by Navy Seals in May. In his notes, Bin Laden speculated about a symbolic attack to mark the anniversary.

But while counter-terrorism officials have reportedly scanned intelligence with extra vigilance during the lead-up to this Sunday, they have found nothing credible until now. Meanwhile, there has been considerable speculation among terrorism experts that al-Qaeda's power and influence are on the wane.

So far this year, there has been less terrorist violence associated with al-Qaeda than in any other year since 2000, said Jason Burke, writing in the Guardian yesterday, before the New York alert was issued.

"Al-Qaeda has been hollowed out," a British security source told Burke. "Of the 10 top people we were most interested in a few years ago, only one is still alive. Maybe we will look back at the first decade of this century as an aberration when a small number of people threatened a superpower."

But others warn that even if al-Qaeda is crippled as a global force, it does not mean that individual jihadists are not eager and capable of launching violent attacks.

The European Union's criminal intelligence agency estimates that less than a third of those arrested for Islamist terrorism in 2010 in Europe were linked to a specific group.

Michael Leiter, director of the US National Counter-terrorism Centre, said the many plots uncovered in America last year were "unrelated operationally but … indicative of a collective subculture and a common cause that rallies independent extremists to want to attack the homeland".

While some US media are reporting today that the current threat emanates from al-Qaeda, Janice Fedarcyk, head of the FBI's New York office, refused to be specific. She told reporters the FBI was aware that "individuals who may be inspired by [al-Qaeda] are often drawn to that ideology and may want to take action. So without calling attention to any specific group, I'll just leave it at that."

What next?Mayor Bloomberg, announcing New York's heightened security, appeared determined to keep his cool in the face of the "credible threat" and urged New Yorkers to be vigilant but not to over-react. He would be taking the subway to work on Friday morning as normal, he promised.

Meanwhile a counter-terrorism official quoted in the New York Times said US intelligence agencies were urgently pursuing leads overseas in an effort to gauge the seriousness of the threat. "It's in the early stages, and a lot of vetting is being done in other places." ·