Analysis of information sources in references of the Wikipedia article "Timeline of the 2015 European migrant crisis" in English language version.
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 [...] 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 [...] are thought to have crossed clandestinely from Syria into Europe.
{{cite news}}: CS1 maint: multiple names: authors list (link)The French prime minister, Manuel Valls, said some of the killers had taken advantage of Europe's migrant crisis to "slip in" unnoticed.