It’s being called a colossal intelligence failure, one that should not have occurred in a country on high alert ever since a major terrorist attack only 10 months earlier.

French police should have caught wind of the planning for Friday night’s attacks, security analysts said, not the least because of their expanded surveillance powers, but also the news that at least one of the French attackers had been identified as being at risk to radicalize.

“We knew for months, especially after the Charlie Hebdo attacks, the question was not if but when we would have another terrorist attack,” said Ariane Chebel d’Appollonia, a researcher at Paris’s Center for Political Research.

But intelligence services have been overwhelmed by the work of identifying those who have left to fight in Syria and those who have returned, she said.

Data from the International Centre for the Study of Radicalization and Political Violence shows that France has sent more fighters to Syria and Iraq than any other European country. Between 1,200 and 1,450 fighters in the region are French, making up a third to half of all foreign fighters, the data show, and as many as 200 have returned home.

News that a Syrian passport found on one of the attackers can be traced to a migrant who arrived in Greece in October has fuelled speculation linking the attacks to the wave of Syrian refugees arriving in Europe. If true, it exposes the weaknesses of the EU’s passport-free Schengen Area, Chebel d’Appollonia said, and a lack of intelligence sharing across the bloc.

“Schengen was to balance the suppression of internal border controls with tougher measures at the external borders. It was not designed to handle the refugee crisis,” Chebel d’Appollonia said. “But even if we want to close the borders, it wouldn’t be enough.”

Even though France has only taken in about 4,500 Syrian refugees since 201l, Chebel d’Appollonia fears they will be blamed. It’s a concern shared by Jean-Pierre Dubois, former president of the Human Rights League in France.

“This has nothing to do with the wave of immigration,” he said. “The same thing would have happened if we hadn’t welcomed a single refugee.”

The only thing the attack has in common with the refugees, Dubois said, is their cause: war in the Middle East.

The attack’s timing was likely prompted by the recent Kurdish victory over ISIS in Sinjar, Iraq, said Dominique Moisi, a founder of the French Institute of International Relations.

“We have to accept the fact that we are at war,” Moisi said, adding “there’s no perfect defence against terrorism.”

The next question for France is whether the UN climate summit, scheduled to start in two weeks, can go ahead as planned.

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“It’s going to be a difficult choice for Hollande,” said Olivier Todd, former editor-in-chief of the French magazine Nouvel Observateur. “If he cancels the meeting, he is admitting that security services can’t prevent attacks by jihadis. But if he allows people to come and anything goes wrong, it’s going to be terrible. It’s a catch-22.”