Paris terror attacks: How did European militants who attacked Paris return from Syria undetected?

As waves of refugees from Syria converged on Europe this summer, law enforcement authorities feared this scenario: that terrorist operatives would slip in among the multitudes, allowing killers and bombers to crash the gates of Europe.

Last week's attacks on France may have validated some of those fears. At least three of the seven known attackers and the suspected ringleader, Abdelhamid Abaaoud - who was confirmed dead Thursday - are thought to have crossed clandestinely from Syria into Europe.

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But there's a catch: The French rampage did not appear to involve Syrian attackers, but rather Europeans who had gone to Syria and returned radicalised and battle-ready to wreak havoc in their homelands.

"We have changed the paradigm," said Jean-Charles Brisard, a terrorism expert. "Our own citizens come back to Europe to perpetrate attacks. Europe needs to take that into account."

More than 3,000 Europeans have travelled to Syria and joined Islamic State and other radical groups, experts say. Their European passports and knowledge of Western society are key assets for those plotting terrorist activities in their homelands, experts say.