When Mehdi Nemmouche walked into the Jewish Museum in Brussels last May and opened fire, he did more than kill four people - including two Israeli citizens. He refocused an intense debate about the open borders within the European Union.
Nemmouche, a French citizen, had only recently returned from Syria, where he had been fighting with jihadi groups. He had a violent past and had served five years in a French prison, where he was apparently radicalised. Despite this background, he was able to cross freely from France into Belgium without any border checks.
The reason for this, of course, is the Schengen Agreement.
First enacted in 1985, later modified and expanded, Schengen became a reality in 1995 in a core group of five countries, including France and Belgium.
Since then, it has become an essential part of the harmonisation of the European economy. The time-consuming and economically costly roadside border checks between member states have completely disappeared. Lorries with manufactured goods move swiftly from the Iberian Peninsula to Germany, or from France across Belgium, the Netherlands, Germany and into Poland without ever having to stop. Tourists, too enjoy the pleasure of motoring from north to south and east to west without hassle.
France identifies jihadi
A global manhunt was under way this week after authorities in Paris identified one of the jihadis who appeared in an Islamic State (IS) video that showed the beheadings of an American aid worker and 18 Syrian soldiers.
The French IS fighter was named as 22-year-old Maxime Hauchard, who converted to Islam at the age of 17.
The Guardian reported that a second Frenchman, who like Hauchard converted to Islam and left France in August last year for Syria, may also have carried out beheadings.
Both men are being investigated for murder with an organised gang and associating with terrorists.
The number of French nationals who have gone to Syria or Iraq to join IS had reached an unprecedented level, according to French authorities. A total of 1,132 people are suspected of joining terrorist organisations.
Hauchard, who comes from a village in Normandy, has been known to French intelligence services since 2011.
He travelled to Syria via Turkey where he claimed to be a humanitarian worker. Once in Syria, he took on the nom de guerre Abu Abdallah el Faransi.
Now, with jihadism presenting a new security challenge, some in Europe are saying that it is time to rethink Schengen.
The Financial Times recently quoted German Interior Minister Thomas de Maiziere calling for changes. He said: "The information that someone is a so-called foreign fighter needs to be more easily recognisable for border authorities."
The article also quoted an unnamed senior counter-terrorism official, who said: "To try and find a way to properly track all of these foreign fighters is already very hard. The lack of information because of Schengen makes it nearly impossible."
But it is not just Schengen that allows the human time bombs trained and primed in places like Syria and Iraq to roam freely. Nemmouche allegedly left the battlefield through Syria's porous border with Turkey, headed north to Istanbul, flew east to Malaysia, on to Singapore and thence to Hong Kong. He then doubled back West and entered the European Union in Frankfurt. Next, he returned to France before setting out on his murderous mission to Brussels. At no stop was he questioned, nor were authorities in Europe alerted.
Others who study the problem of terrorism inside Europe's open borders are more sanguine about the situation than Mr de Maiziere. Raffaello Pantucci, of the Royal United Services Institute, notes that terrorism was part of the European scene when Schengen was first being implemented. The IRA and ETA, the Basque separatist group, moved around freely even when the borders were closed. Built into Schengen is the right of national security forces to close borders and perform checks if they have a suspicion about terrorist activity.
The problem today, says Mr Pantucci, is "the scale and flow of terrorists is novel. You have countries dealing with the jihadi problem who have not dealt with it before."
The real problem, in his view, isn't Schengen, it is that "we don't have common European police, laws and courts". This means that each country commits the resources it deems necessary to tracking real and potential jihadists among its citizens.
In Britain - which has seen an estimated 500 young men go off to Syria and has a wider group in the Muslim community who at least understand their reasons, if not feel sympathy for them - this is a major problem, with appropriate resources spent monitoring the comings and goings of jihadists. Portugal might have no more than a handful of potential recruits to Islamic State (IS) and so gives tracking them fewer resources.
The German Interior Minister's call to rewrite Schengen has not found much support in Brussels. Pieter Cleppe, head of the Open Europe think tank, said there had not been much criticism of Schengen following the shootings from Brussels security chiefs. "The far right, obviously, is against it," he points out. "But if you close borders you will annoy, honest, hardworking people. Besides, the criminals would find a way to cross anyway."
Both men point to the lack of co-ordination among the security services of the EU member states as being the main problem in dealing with the potential havoc that could be wreaked by the return of Europe's estimated 3,000 men and women fighting with IS and other groups.
"By their nature, the covert services are secretive and they don't share info with each other and frequently don't share info that can be used in a court of law," said Mr Pantucci. "Euro-federalism might ameliorate the situation but federalism is a toxic subject."
And even a federal Europe might still have problems. The US, which is a federal system, is notorious for lack of co-ordination between security agencies. Turf wars break out all the time. The FBI withholds information from state law enforcement and vice versa. Even within agencies there is a reluctance to share. Famously in early 2001, the FBI field office in Phoenix, Arizona was tracking some young Saudi men taking flying lessons and raised questions that somehow never got passed along to the highest levels for investigation. The result was the 9/11 attack.
Open Europe's Cleppe thinks that there are tools already available to catch extremists. He notes police can close borders occasionally and they do track other kinds of criminal activity. In fact, that is how Nemmouche was caught.
After murdering the four people at the Jewish Centre in Brussels, he headed to Amsterdam. There he got on a bus from Amsterdam to Marseilles.
What he did not know was that the bus is regularly used by small-time drug dealers who buy narcotics in Amsterdam and try to carry them into France. The police are well aware of this and frequently inspect the bus.
When Nemmouche got to Marseilles, the police were carrying out one of those inspections but instead of drugs, they found a small arsenal on the terrorist, including a revolver, a kalashnikov and a couple of hundred rounds of ammunition.
Once arrested in France, Nemmouche was immediately extradited to Belgium on a European Arrest Warrant (EAW), Mr Cleppe points out. He is too polite to say that the EAW - seen in some in the UK as a deeply flawed law - is the latest stick British euro-sceptics have been using to bash an overpowerful Brussels in their relentless drive to get Britain out of the EU.
The calls to rewrite Schengen will not go away. The next time there is an atrocity by a jihadi returning from Syria, they will echo loudly around the continent. Mr Pantucci says that there is not much that can be done. "About the only thing you could do is to strengthen Schengen's outer borders." In other words, put tighter checks along the Mediterranean from Turkey to the Straits of Gibraltar.
And even then, it is the nature of this particular strain of terrorism, so different from that practised by the IRA and ETA, that the authorities cannot do much in the way of prevention.
"You never totally know who might turn into a murderer," said Mr Pantucci. "There are potentially a lot of people. Many have indicators and never act on them. It's an indication of the magnitude of the problem."
