By John Irish, Robert-Jan Bartunek and Orhan Coskun
PARIS (Reuters) - There were multiple chances to stop the men who attacked Paris.
In January, Turkish authorities detained one of the suicide bombers at Turkey's border and deported him to Belgium. Brahim Abdeslam, Turkish authorities told Belgian police at the time, had been "radicalized" and was suspected of wanting to join Islamic State in Syria, a Turkish security source told Reuters.
Yet during questioning in Belgium, Abdeslam denied any involvement with militants and was set free. So was his brother Salah – a decision that Belgian authorities say was based on scant evidence that either man had terrorist intentions.
On Nov. 13, Abdeslam blew himself up at Le Comptoir Voltaire bar in Paris, killing himself and wounding one other. Salah is also a suspect in the attacks, claimed by the Islamic State, and is now on the run.
In France, an "S" (State Security) file for people suspected of being a threat to national security had been issued on Ismail Omar Mostefai, who would detonate his explosive vest inside Paris' Bataclan concert hall. Mostefai, a Frenchman of Algerian descent, was placed on the list in 2010, French police sources say.
Turkish police also considered him a terror suspect with links to Islamic State. Ankara wrote to Paris about him in December 2014 and in June this year, a senior Turkish government official said. The warning went unheeded. Paris answered last week, after the attacks.  
A fourth attacker missed at least four weekly check-ins with French police in 2013, before authorities issued an arrest warrant for him. By that time he had left the country.
On any one of these occasions, police, intelligence and security services had an opportunity to detain at least some of the men who launched the attacks. 
That they did not, helps explain how a group of Islamist militants was able to organize even as they moved freely among countries within the open borders of Europe's passport-free Schengen area and beyond.