On any one of multiple occasions, police, intelligence or security services had an opportunity to detain at least some of the men who launched the attacks on Paris.

That they did not helps explain how a group of Islamist militants was able to organize as they moved freely between countries within the open borders of Europe's passport-free Schengen area and beyond.

In January, Turkish authorities detained one of the suicide bombers at Turkey's border and deported him to Belgium. Turkish authorities told Belgian police at the time that the man, Brahim Abdeslam, had been "radicalized" and was suspected of wanting to join Islamic State in Syria, a Turkish security source told Reuters.