The thwarted attack by a gunman on a high-speed train between Amsterdam and Paris has underlined the difficulty faced by intelligence services in tracking the unprecedented numbers of potential jihadists, experts say.
The Moroccan suspect in Friday`s incident, identified as 25-year-old Ayoub El Khazzani, was first flagged by Spanish authorities as a potential extremist and had reportedly travelled to Syria.
He was overpowered by two US servicemen and other passengers before he could kill anyone in the train.
A Spanish counter-terrorism source said Khazzani had lived in Spain from 2007 to March 2014, before travelling to Syria from France.
In a timely interview published on the same day as the attack, Alain Grignard, a senior member of Belgium`s counter-terrorism police unit, said the terrorist threat has "never been higher in all the years I`ve been working".
"It boils down to mathematics and it`s all linked to the Syria dynamic," Grignard told CTC Sentinel, the in-house journal of the US military academy at West Point.
"There`s no way of knowing the exact numbers but I can tell you with certainty that at least 300 have travelled -- that`s the number we have sufficient evidence to bring charges against. At least 100 have returned to Belgium, but we are under no illusions that there aren`t more we don`t know about. It`s impossible to do surveillance on everybody."
It`s a point driven home by terrorism expert Raffaello Pantucci, of the Royal United Services Institute in London.
"The authorities know about a lot of people but not which ones will actually launch an attack," he said.
"It`s a very resource-intensive job. You need three shifts with several people, and equipment and vehicles, to watch someone 24 hours a day. Intelligence agencies just aren`t big enough to do that for everyone."Grignard said the number of terrorism suspects had mushroomed in recent years.
"To give you an idea of the scale of the challenge, in the past two years we`ve charged more people with terrorism offences than in the 30 years before that," he said.
"Our approach in Belgium is to detain everybody suspected of fighting with terrorist groups in Syria when they return to Belgium. But in lots of cases we do not have enough evidence (to charge them)."
Belgium, where Khazzani boarded the train in the capital Brussels, is thought to have the highest per capita number of people in Europe leaving to fight in the Middle East.
The threat was underlined when a jihadist cell linked to the Islamic State group was busted in Belgium in January, thwarting an alleged plot to attack police.
But France has the highest overall numbers joining the jihad, with the government reporting that 843 had left for Syria as of May -- more than half of them unknown to authorities at the time of their departure.
"It`s the perennial problem of how you prioritise between serious concerns," said Pantucci. "Plus there`s the Schengen free movement -- it`s very easy for people to move around.
"Intelligence agencies in different countries are getting better at talking to each other, but they may make different assessments of individuals and put their priorities in different areas."
As if to underline the obstacles to intelligence-sharing, France and Spain have issued conflicting accounts about Friday`s gunman, with Spanish authorities telling AFP he travelled from France to Syria and back last year, while French officials say they were never informed of this. Although jihadist profiles vary greatly, one of the key trends in Grignard`s view has been an increasing number who come from "inner-city gang" backgrounds.
"Young Muslim men with a history of social and criminal delinquency are joining up with the Islamic State as part of a sort of `super-gang`," he told CTC Sentinel.
He cites the cell broken up in January "who were radicalised very quickly, and when they came back from Syria they had no fear of death". Two militants were killed in the raid.
"When our commandos launched their raid it took the suspected terrorists one second to switch from chatting between themselves to opening fire.
"These guys had maybe more experience in gun battles than our own commandos. Here in Belgium and across Europe we are now reviewing how we do these kind of raids," said Grignard.
There is also a limit to how much security can be mounted, particularly around trains.
Belgium said Saturday it would increase baggage checks and patrols on high-speed trains, while France will introduce an emergency hotline to report "abnormal situations".
But with 3,000 stations in France alone, absolute security is impossible.
"Airplanes leave from a specific place -- you can build a security apparatus around it," said Pantucci. "It`s just not possible to do that with trains. You would have to do that at every station from large terminals in Paris to small towns in rural France."