There are plenty of reasons why: jihadi networks hiding among the millions of Muslim immigrants living in Europe, travel by would-be jihadists to and from the killing fields (and training grounds) of Syria and Iraq, as well as easy movement within Europe. Flights may be secure, but rail and other transport hubs in Europe are not. There are plenty of targets. And ready access to weapons such as AK-47s make basic acts of terrorism (rather than ambitious bomb plots) easy to plan and carry out.
In addition, cooperation among European intelligence services -- while improving -- is not as thorough as it should be. It also remains extremely difficult to foresee who among the many thousands of radicalized individuals will cross the line from thought to action -- monitoring them all would require massive resources and a level of surveillance that's difficult to reconcile with democracy.
There's now another worry: are groups like ISIS trying to smuggle operatives into Europe among the tens of thousands of migrants crossing the Mediterranean? One prominent ISIS follower, Abu Arhim al-Libim said in February
that even if the migrant exodus from Libya was "partially exploited and developed strategically, pandemonium could be wrought in southern Europe."
Ayoub el Khazzani is still being questioned in France by the DGSI -- the domestic intelligence and security service. He has not been charged. But what's emerging about his past illustrates many of the problems European intelligence agencies face.
Large marginalized population
According to Spanish officials, the Khazzani family moved to Spain in 2007. They were originally from Tetouan, a town of high youth unemployment on Morocco's Mediterranean coast and long a recruiting ground for jihadist groups. It was then very common for Moroccans and other foreigners to be granted residency and work permits in Spain. The number of migrants living in Spain increased from less than 2% of the population in 1999 to more than 12% a decade later.
The family later moved from the capital, Madrid, to the port city of Algeciras just 10 miles across the Mediterranean from the Moroccan coast. They lived in the neighborhood of El Saladillo, which suffers from poverty, high crime and drug trafficking. In April of this year, for example, Spanish police intercepted 850 kilograms of Moroccan hashish being unloaded at El Saladillo's harbor.
Khazzani was arrested at least twice on drug-related charges in Madrid, according to Spanish accounts of his background. His father, interviewed
by the Daily Telegraph Sunday, said his son's first arrest was in 2009. He appears to have survived on odd jobs and occasional petty crime, much like Mohammed Merah -- who went on a shooting rampage in Toulouse, France in 2012, killing seven people -- and Mehdi Nemmouche, who returned from Syria and carried out a gun attack against a Jewish museum in Brussels in May 2014.
The link between petty crime or drug addiction and then redemption through militant Salafism -- a literalist approach to Islam that seeks what its followers consider a return to the true values of the religion -- has surfaced time and again in terror investigations, with prisons frequently the incubator for such transformations. European governments are grappling with the problem of combating radicalization in prisons. After the January attacks in Paris, the French government doubled the budget for Muslim chaplains in prisons. But the task is daunting: more than half the male population of French jails is Muslim. Amedy Coulibaly, who attacked a Jewish supermarket in Paris in January, told police after a previous arrest that he had met many terrorists in prison.
Ayoub el Khazzani began attending a mosque in Algeciras after he reunited with his family in 2012 and, according to locals in El Saladillo quoted in Spanish media, became more devout. What he did to attract the attention of the Spanish authorities is unclear. But according to French Interior Minister Bernard Cazeneuve, the Spanish police alerted the French that Khazzani was preparing to travel there early in 2014. Beyond that, there appears to be disagreement between French and Spanish sources about who knew what and when and what was communicated.
Close cooperation among intelligence services is all the more critical because of free movement within the European Union's Schengen Area, which allows for passport-free travel between 26 countries. Speaking of Khazzani, Belgian Prime Minister Charles Michel said "this was someone who clearly traveled around Europe." Khazzani's lawyer, Sophie David, said he had visited Germany and Austria as well as France and Belgium.
Such travel patterns are not uncommon: terror groups have learned to use Europe's open borders to their advantage. The French commentator Gabriel Robin wrote Sunday that "the utopia of a world without frontiers or national identities, or more simply without different nations, has become our nightmare."
In suggesting more identity checks, Michel said Schengen was important for "freedom of movement for those who have good intentions, but this freedom is also used in order to harm."
German Interior Minister Thomas de Maiziere has suggested Schengen may be at risk because of the refugee crisis unfolding at its borders. But the agreement would be very difficult to unravel, and European Commission President Claude Junckers has said it is irreversible. To him and others, Schengen's failure would be a defeat for democracy and an open Europe, whether inflicted because of terrorism or Europe's inability to share the refugee burden.
Weapon of choice
Khazzani had an AK-47 when wrestled to the floor of the train. Sid Ahmed Ghlam, a 24-year-old Algerian arrested in Paris in April after killing a woman, had several in a vehicle. The weapon was allegedly supplied to him by a cell with links to jihadist groups in Iraq and Syria -- and prosecutors contend he was planning terror attacks against one or more churches. Amedy Coulibaly also used an AK-47, as did his accomplices -- the Kouachi brothers -- who attacked the Charlie Hebdo offices (and also had a Skorpion submachine gun) the previous day.
Such weapons are not difficult to come by in industrial cities on either side of the Franco-Belgian border, according to intelligence analysts, nor in other big cities such as Grenoble and Toulouse. Many are smuggled from the Balkans, after being stolen or bought from corrupt officials. Some of the world's largest weapons and ammunition manufacturers are based in the region, which was flooded with guns during the collapse of Yugoslavia in the 1990s.
According to a recent European Union report
, "large amounts of powerful military grade weapons have since the mid-1990s reached the EU from the Western Balkans and former Soviet Bloc countries, often trafficked in small quantities and hidden in vehicles like long-distance coaches to avoid detection."
Another report estimated that in 2011 there were 100,000 tons of ammunition stockpiled in Bosnia alone.
Who To watch?
Sid Ahmed Ghlam was on the radar of the DGSI, having been cautioned by police after returning from Turkey. He and Khazzani -- and several thousand others -- were included in what's known as the 'fiche S' by the DGSI. This huge and fluid list categorizes individuals according to 16 levels of assessed threat to national security. But many who have spent time on the list have gone on to commit terror attacks: Nemmouche, Coulibaly and the Kouachi brothers among them. Mohammed Merah was interviewed several times by the security services before he launched his attacks in Toulouse. Yassin Salhi, accused of beheading his employer in June, had been removed from the fiche S in 2008.
But constant surveillance of all those moving in militant circles or perceived as a potential threat is impossible, as is predicting which among them may go 'operational.' There may be as many as 5,000 individuals in the fiche S. Security experts say that full-time surveillance of just one individual can involve 25-30 agents. And some suspects appear to "go straight" for a time in an effort to shake off surveillance, or use wives or partners to communicate by phone (a ploy used by Coulibaly and one of the Kouachis.)
Investigators will now be focusing on whether and when Khazzani might have traveled to Syria or Iraq, or even Turkey. His actions on the train, as reported by witnesses, do not suggest a highly-trained assassin. But security services are perennially anxious that ISIS may seek to expand its war by training and sending some of its foreign fighters back to Europe to wreak havoc.