Washington (AFP) - The Islamic State group has brought down a crowded airliner over Egypt, shot up Paris cafes and other soft targets, and inspired attacks like the shooting rampage last week in California.

And experts say the West is and will remain vulnerable to the threat, for the foreseeable future.

IS may not have exerted direct control over all these attacks. But it inspired them, welcomed them and glorified them, analysts say.

That is unlikely to change any time soon, as thousands of foreign jihadists swell the group's ranks in Iraq and Syria with the idea, at least for some of them, of coming home seasoned and even more deeply radicalized.

"Every European security service that I have talked to in the last year is petrified of the issue of foreign fighters, and there is almost no solution to it," said Bruce Riedel, a former CIA official who now works at the Brookings Institution.

The Soufan Group, an intelligence consultancy, released a report Tuesday that said the number of foreign fighters who have travelled to Iraq and Syria since 2011 is thought to have reached 27,000 to 31,000, from at least 86 countries -- double the estimate from a year ago.

"Think about the problem of just monitoring people when they come back. We're going to put them all in jail. Most of them will be out within 48 hours, because they did not commit a crime in France or in Belgium," said Riedel, speaking of Europe's returnee fighters.

"And it's very hard to prove that they committed a crime in Syria or Iraq. Do they have the intention to commit a crime? Possibly. But you can't keep people in jail in democracies because they are suspected to have the intention of committing a crime," he added.

Close monitoring of these thousands of suspects would require around the clock work by virtually the full manpower of all western security and military forces, he added.

"It's impossible to do. So we have a serious problem: it's what I call task saturation," Riedel added.

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Besides jihadists returning home from the Middle East, the Islamic State group can rely on an unknown number of supporters like the married couple that shot dead 14 last week in San Bernardino, California.

These kind of people turn radical without having direct contact with the Islamic State group and without drawing much attention to themselves before attacking.

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David Kilcullen, a counter-terrorism expert who took part Tuesday with Riedel in a terrorism conference in Washington, calls this "leaderless jihad."

"Instead of having a clandestine structure with two-way communication and secret plans that need to be protected, you have an open communication structure that is broadcast-based, with people who communicate openly with people who are underground and who act in general terms, in accordance with the guidance," said Kilcullen.

"The possibility of the melding of foreign fighters returning to their homeland with the existence of the underground network in various parts of the EU in particular changes significantly the domestic threat," he added.

The December 2 mass shooting in San Bernardino, in the wake of the Paris attacks last month, shows "the limits of the counterterrorism 'radar' constructed after 9/11 to detect and disrupt network-based attacks."

This radar needs to be "recalibrated," it added.

Terror cells formed by a married couple that keeps to itself, or a group of brothers or childhood friends who are careful to stay away from mosques and off the Internet before striking, are impossible for intelligence services cells to detect or infiltrate.

"There is no computer program that can detect and predict violent radicalization at the granular level of two individuals," said the Soufan Group.

"The odds are in the criminals' favor if they keep quiet and act quickly," it added.

"As impossible as it is to eradicate violent crime, it is impossible to eradicate terrorism, especially attacks planned under the radar," the group added in its study.

"There are lessons to be learned from the San Bernardino attack, and one of them might be that there will be more such attacks in the future, despite our best efforts."