PARIS • There were multiple chances to stop the men who attacked Paris.
In January, the Turkish authorities detained one of the suicide bombers at Turkey's border and deported him to Belgium. Brahim Abdeslam, the Turkish authorities told Belgian police at the time, had been "radicalised" and was suspected of wanting to join the Islamic State in Iraq and Syria (ISIS), a Turkish security source told Reuters.
Yet during questioning in Belgium, Brahim denied any involvement with militants and was set free. So was his brother Salah - a decision that the Belgian authorities said was based on scant evidence that either man had terrorist intentions.
In France, an "S" (State Security) file for people suspected of being a threat to national security had been issued on Ismael Omar Mostefai, who would detonate his explosive vest inside Paris' Bataclan concert hall on Nov 13. Mostefai, a Frenchman of Algerian descent, was placed on the list in 2010, said French police sources.
Turkish police also considered him a terror suspect with links to ISIS. Ankara wrote to Paris about him in December last year and in June this year, said a senior Turkish government official. The warning went unheeded. Paris answered last week, after the attacks.
We're in a situation where the services are overrun. They expect something to happen, but don't know where.
MS NATHALIE GOULET, who heads the French Senate's investigation committee into Islamist networks
A fourth attacker, Samy Amimour, missed at least four weekly check-ins with French police in 2013, before the 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 were able to organise even as they moved freely among countries within the open borders of Europe's passport-free Schengen area and beyond.
Taken one by one, each misstep has its own explanation, security services said. They attribute the lapses in communication, inability to keep track of suspected militants and failure to act on intelligence, to a lack of resources in some countries and a surge in the number of would-be extremists.
But a close examination by Reuters of a series of missed red flags and miscommunications culminating in France's biggest atrocity since World War II puts on stark display the mounting difficulties faced by anti-terrorism units across Europe and their future ability to keep the continent safe.
The man identified as the ringleader, Abdelhamid Abaaoud, was himself well-known to the authorities for several years. After a police raid in January to nab him in the Belgian town of Verviers, Abaaoud escaped to Syria, but then made his way back into Europe at some point after January. The French authorities did not know this until they were tipped off by Morocco after the attacks.
"If Abaaoud was able to go from Syria to Europe, that means there are failings in the entire European system," French Foreign Minister Laurent Fabius said.
Bilal Hadfi, who blew himself up outside the Stade de France, was another suicide attacker under surveillance after visiting Syria in February.
Police are still looking for Salah, who is known to have survived the attacks. Until six weeks before the attacks, Salah and his brother Brahim - one of the suicide bombers that night - were running a bar called Les Beguines on a quiet street in Molenbeek, a low-rent area of Brussels which has been linked with several attacks.
After the attacks, Salah went to ground. The authorities said that he was stopped on his way back to Belgium after the Paris attacks, but that police waved him on. It is not clear what role he played on the night of the attacks and why he managed to survive.
Missteps did not just happen in France and Belgium. The Syrian passport found near one of the suicide bombers at the Stade de France had been used by a man registering himself as a refugee on the Greek island of Leros on Oct 3.
That man travelled through Macedonia and claimed asylum in Serbia, counter-intelligence and security sources said. The French prosecutor has confirmed that fingerprints taken on arrival in Greece showed that the man travelled with a second man, who also blew himself up near the Stade de France.
"We're in a situation where the services are overrun. They expect something to happen, but don't know where," said Ms Nathalie Goulet, who heads the French Senate's investigation committee into Islamist networks.
Determining how dangerous a person is, and whether such an individual might carry out an attack, is a key challenge for security services, said experts.
"The other difficulty is that if you have nothing concrete for several years, you can't keep either a sophisticated technical alert system or human resources on a person who makes himself forgotten for three or four years," said former intelligence officer Arnaud Danjean, who is now a member of the European Parliament.