Another week another terrorist attack, this time in London. As the Islamic State (IS) group claims responsibility for Friday’s rush hour Tube bombing in Parsons Green, so the UK terror threat was raised to critical, its highest level.

The suggestion is that another strike may be imminent, even though a man was arrested yesterday in the port area of Dover in connection with Friday’s bombing.

The latest attack has once again begged the question as to just precisely who the perpetrators are behind the bombings, stabbings, and vehicle-borne rampages that have haunted Europe of late - the first real violent manifestation of foreign fighters returning from the battlefields of Iraq and Syria.

Just days before the latest attack in London, thousands of miles away on the frontlines of Syria, Kurdish forces took custody of an American citizen, who is said to have either been captured or surrendered after fighting for IS.

The American fighter, whose identity was not released and whose whereabouts are currently unknown, is not the first American IS fighter ‘captured’ on the battlefield.

But it is another reminder of the many foreign fighters who joined the ranks of the terrorist group over the past few years, some of whom are now seeking to return to their countries of origin.

Many appear disillusioned with the organisation that once promised them a caliphate but now finds itself in retreat across the Middle East.

Others though have more sinister reasons for leaving the frontlines of the region, under orders or seeking to bring their battlefield skills and violence to the doorstep of the European continent.

Right now on the Syria-Turkey border, several dozen fighters are said to have already made it across the heavily patrolled frontier to towns and cities in Turkey’s south in recent weeks.

Hundreds more defectors from IS have also massed in Syria’s Idlib province, doubtless many planning to cross the nearby Turkish border and find ways back to the Middle East, North Africa, Europe and beyond from where they originate.

With the retaking of IS strongholds in Mosul and Tal Afar in Iraq and the noose tightening around others in Iraq and Syria like Hawija and Raqqa, the exodus of foreign fighters has begun in earnest.

To say that some officials within the intelligence community are seriously worried about this return flow would be an understatement.

Just a few months ago EU Commissioner for the Security Union, Julian King, warned that the re-taking of IS strongholds could lead to a scenario in which violent militants would return to Europe.

“This is a very serious threat and we must be prepared to face it,” King emphasised.

EU Counter-Terrorism Coordinator Gilles de Kerchove likewise cautioned that the EU would struggle to handle the predicted 1500-2000 foreign fighters that might head back towards Europe.

One of the biggest problems facing those tasked with making an assessment of the threat posed by returning fighters is establishing a realistic tally of numbers.

According to a detailed report complied by the International Centre for Counter-Terrorism (ICCT), entitled the Foreign Fighters Phenomenon in the European Union, the EU wide estimate lies between 3,922 and 4,294 foreign fighters.

A majority of these, around 2,833, come from just four countries: Belgium, France, Germany and the UK with Belgium having the highest per-capita foreign fighter contingent.

In all, as of April this year, the ICCT research suggests that an average of 30 per cent have already returned to their countries of origin or departure equalling about 1,200 people.

But as Islamist inspired terror attacks across Europe increase, many security officials see this as the first indication of the nightmare scenario they feared, whereby IS leaders send their foot soldiers back to launch attacks across the West.

The past years have seen several attacks connected to foreign fighters. These include the January 2015 attacks on the headquarters of the satirical newspaper Charlie Hebdo, and the subsequent attack on a kosher supermarket in Paris, as well as an earlier attack by a French national, who had allegedly spent several months fighting in Syria before carrying out an assault on a Jewish museum in Brussels in May 2014.

But it was not until the tragic events that unfolded on the night of November 13 2015 in the streets of Paris that fears of a large-scale attack involving groups of returnees from Syria/Iraq were painfully confirmed. At least seven of the perpetrators were alleged to have fought with IS. The most recent attacks in Barcelona have once again highlighted concerns over sleeper cells and connections with foreign fighter networks.

While up until now the focus for the European security and intelligence agencies has primarily been on preventing the travel of potential foreign fighters to the battlefields of Iraq and Syria, currently they now have to extend their operations to addressing the return of the jihadists.

“Security agencies across Europe face an almost unprecedented task,” says Dr. Shiraz Maher, a lecturer at King’s College London and the author of Salafi-Jihadism: The History of an Idea.

“The scale of foreign fighter mobilisation since 2012 into the Syrian conflict has been so extreme that no one really knows how to deal with it. The agencies are stretched to breaking point, responding to plot after plot,” said Maher recently.

With jihadists again on the move this time in the opposite direction, a situation already challenging has become even more complex.

Among those terrorists, western intelligence agencies believe, are prominent members of IS’s external operations arm, who joined the group from numerous European countries including Britain, France and Belgium, as well as Australia.

At least 250 ideologically driven foreigners are thought to have been smuggled back into Europe from the Middle East from late 2014 until mid-2016, with nearly all travelling through Turkey after crossing a now rigidly enforced border.

France and Belgium, scenes of some of the deadliest IS-inspired attacks in recent years, as well as Italy have taken steps to track returning extremists and block their return whenever possible.

Authorities in all three countries privately admit that they have little interest in helping their citizens who joined IS return home.

“We are happy to let the Iraqi legal process handle crimes committed by any French citizens who joined Daesh,” a French Ministry of Defence official said, using the Arabic acronym for IS. “But ideally most of the worst will be handled on the battlefield.”

The official’s dismissive attitude is perhaps indicative of the unprecedented strain on much of Europe’s security infrastructure from years of homegrown plots and threats that at times it has struggled to cope with.

Belgian and French officials have confirmed that their intelligence agencies have sent agents to both Syria and Iraq to quiz hundreds of jihadist fighters now in captivity and the thousands of their family members housed in refugee camps to determine what’s become of those who went to fight with IS.

“It’s a nightmare to keep track of all these guys who might be dead, or might have a new name, or might want to come home,” said one Italian law enforcement official.

“And the dirty secret here is how all these pro-human rights European governments are forced to get into bed with Arab despots and killers to get help tracking all these guys.”

Profiling, identifying, tracking and locating foreign fighters who have moved into and out of Iraq and Syria remains a challenge. According to intensive ICCT research there is no clear-cut profile of a European foreign fighter.

Based on the responses of several European member states, the indications are that around 90 per cent originate from large metropolitan areas or peripheral suburbs. Many too originate from the same neighbourhood, which seems to indicate that there are pre-existing extremist networks operating in these areas, that a circle of friends radicalise as a group and decide to leave jointly for Syria-Iraq.

Then there is the question of nationality with no clear pattern emerging regarding foreign fighters. In some countries the majority hold a nationality other than the one of the country where they departed from, whereas in other countries the opposite trend can be observed.

ICCT research also show a sizeable number of converts to Islam and based on data from eleven EU member states an average of 17 per cent are female.

Given this complexity, the fusion between homegrown extremist and foreign fighters only adds to their layers of operational activity. For the security services it poses a two pronged threat.

Alongside those battle-hardened jihadis back from Iraq and Syria there is an even larger army of “sleeper” extremists in towns and cities across the European continent. These violent wannabe jihadis may lack the skills of trained fighters, but are just as determined to inflict their brand of terror often on their own communities.

Most of these radicalised individuals, 3,500 in the UK alone, have never even been to the Middle East. They learned their deadly craft online, and what many security analysts fear is that returning foreign fighters, while themselves not directly operationally active, will act as mentors to these home grown terrorists.

Faced with such challenges officials here in Britain are engaged in reviewing their strategy in the wake of multiple terror attacks in London and Manchester in the early summer.

Security sources believe the number of potential returnee foreign fighters remains more than double the 150 suspects known to have been stripped of citizenship for terror activity abroad in recent years.

There are believed to be at least 350 British citizens still active in the Iraq-Syria region, but primed to come back.

Intelligence analysts believe another 400 British citizens have already returned and while some are under close surveillance, others have slipped through the net.

Addressing the figures, Ben Wallace, the Home Office minister for security, hinted recently that a wide range of measures were being rolled out to "disrupt and diminish" the threat as IS loses territory.

Not all foreign fighters will of course opt to return to their countries of origin. “The European intelligence services, starting with MI5 and MI6, now have lists focused on secondary territorial movement,” says Matteo Toaldo, a researcher at the European Council on Foreign Affairs in London. According to Toaldo, Libya remains “a plan B destination” for many.

“Parts of Libya are seen as safe havens by foreign fighters who want to continue the battle,” he says.

While IS might be on the backfoot in Iraq and Syria it has consistently shown its capacity to metamorphose and reinvent its strategy to prevailing pressures.

Few doubt that in anticipation of its failure to remain in control of Mosul, Tal Afar, and Raqqa that IS’s own foreign intelligence service Amn al-Kharji would not have drawn up contingency plans to take the fight behind “enemy lines” perpetrating terrorist operations in Europe and elsewhere.

There is precedence here. The model for this kind of scenario was previously played out more than a decade ago, when the most feared terror group at that time, al-Qaeda, felt the full wrath of coalition airstrikes and ground operations in Afghanistan and elsewhere.

Al-Qaeda's leaders at the time urged their followers to strike back, and they duly did, launching attacks in London in 2005 and Madrid, a year earlier.

“Terrorists do not sit still, and nor should we,” Julian King, EU Commissioner for the Security Union, said in the wake of the recent Barcelona outrages.

As the increasing regularity of Islamist inspired attacks shows, IS is set to test just how nimble and effective that European response policy will be for along time to come yet.

Next week David Pratt will be reporting from northern Iraq as tensions rise ahead of the Kurdish independence referendum due to be held on September 25