Has Europe under-estimated the security threat posed by battle-hardened homegrown Islamic militants returning from Syria?
After the chance arrest in France of the suspected perpetrator of last week's deadly shooting at a Jewish Museum in Brussels, concern mounted Monday that the region is ill-equipped to cope with the threat posed by homegrown Islamic militants returning from Syria's battlefields.
Mehdi Nemmouche, a Frenchman who spent more than a year fighting in Syria, was detained on Friday in Marseille following a random customs check on the coach on which he had been traveling, which had started its journey in the Netherlands.
Nemmouche, 29, was carrying a Kalashnikov automatic rifle and a handgun, the same type of weapons used in the museum attack, and a camera which contained a 40-second film in which he appears to claim responsibility for a professionally-executed shooting which left three people dead and another man in a critical condition.
Police sources said Nemmouche had refused to say anything to his interrogators. His lawyer told AFP that, "in principle" he would not oppose request for him to be handed over to the Belgian authorities.
Investigators are trying to establish if he acted entirely alone and if he was planning other attacks, as French Interior Minister Bernard Cazeneuve said was highly likely.
"He was extremely dangerous," Cazeneuve said. "It was probable that he would have struck again."
Questions meanwhile have begun to be raised about how an individual who had been known to the French security services was able to avoid surveillance following his return to Europe.
The DGSE intelligence agency had been notified of his return when he landed in Germany at the end of March.
Radicalized in prison, Nemmouche spent over a year fighting for the Islamic State of Iraq and the Levant (ISIS), the most radical of the jihadist groups in Syria which is closely aligned with Al Qaeda.
Europe can expect further "small-scale attacks" like the fatal shooting at the Jewish Museum in Brussels, the EU's anti-terror chief Gilles de Kerchove said Monday.
"I don't expect another 9/11," he said in a video statement. "I don't expect a major sophisticated attack."
"I [do] expect more small-scale attacks like the one in Brussels," he added.
Not everyone agrees. French Al-Qaeda expert Jean-Pierre Filiu believes the threat posed by groups like ISIS, for Europe, is greater than that created by the Taliban's sheltering of Osama bin Laden in Afghanistan prior to the 2001 attacks on New York.
"Barack Obama said last week that a new September 11 is not possible in the United States. He was probably right because there are only a handful of American jihadists in Syria, but the danger of a European 9/11 is more real," Filiu said.
For many in France, the Nemmouche case has harrowing echoes of that of Mohammed Merah, the Islamist gunman who shot seven people - a rabbi, three Jewish children and three French paratroopers - in the southern city of Toulouse in March 2012.
Like Nemmouche, Merah was on the radar of the French security services who under-estimated the threat posed by an individual who had spent time in Afghanistan and Pakistan.
Marine Le Pen, the leader of the far-right National Front (FN) party which topped the polls in recent European elections, accused the government of "stupefying naivety" in under-estimating the scale of radicalization in France's large Muslim community.
Le Pen said the authorities should be taking measures including the restoration of national border controls in continental Europe, special prisons for jihadists and for convicted militants to be stripped of their French nationality.
Anti-terrorism experts have long warned of the "blowback" threat posed by European-born Islamists returning home with combat experience and the weapons expertise required to carry out terrorist attacks.
De Kerchove said more than 2,000 Europeans had left for Syria or were on their way there, where "they will get military training, become much more radicalized, and have a network across Europe."
Though "few will mount an attack", the Brussels incident was "very, very worrying" for Europe's future security, he said.
De Kerchove said that though it remained to be seen whether or not Nemmouche acted on his own inspiration, "I don't think he is a lone wolf."
The attack however showed the need for European Union nations to coordinate against the threat of jihadist fighters returning home from the Syrian front.
The risk was highlighted in a report to the UN Security Council last week which warned that Syria could prove to be a cradle for new pan-Arab and pan-European networks of extremists.
France and Belgium have agreed to step up cooperation on the issue in the wake of Nemmouche's arrest.
But experts say the comprehensive monitoring of suspects requires huge resources which security services do not currently have.
"When they come back from Syria, they don't always return to their home country, they may be using assumed names and they often have several telephones," Louis Caprioli, who worked for the anti-terrorist wing of France's National Police from 1998-2004.
The large number of individuals who could potentially pose a threat makes it practically impossible to ensure surveillance of all of them and police are forced to concentrate on those deemed to pose the greatest threat, he added.
The willingness of militants to act in isolation, as happened in both the Merah and Nemmouche case, also makes preventative action harder.
"We are dealing with a generation of militants who, while they have been in contact with jihadist groups, are prepared to operate on their own and carry out attacks that require little in the way of equipment but are guaranteed to have a huge impact," said Dominique Thomas, a specialist in Islamist movements in the Arab world.
French police on Monday arrested four men suspected of involvement in the recruitment of jihadists and a related arrest was made near Brussels, but none of the five detained have been linked to Nemmouche.