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 "radicalised" and was suspected of wanting to join Islamic State in Syria, a Turkish security source said.
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 13 November, 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.
Taken one by one, each misstep has its own explanation, security services say. 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 jihadis.
But a close examination of a series of missed red flags and miscommunications culminating in France's biggest atrocity since World War Two puts on stark display the mounting difficulties faced by anti-terrorism units across Europe and their future ability to keep the continent safe.
"We're in a situation where the services are overrun. They expect something to happen, but don't know where," said Nathalie Goulet, who heads up the French Senate's investigation committee into jihadi networks.
Many point to Belgium as a weak link in European security.
"They simply don't have the same means as Britain's MI5 or the DGSI (French intelligence agency)," said Louis Caprioli, a former head of the DST, France's former anti-terrorism unit.
Belgian Prime Minister Charles Michel defended his country's security services and praised them for doing "a difficult and tough job." French President Francois Hollande also praised his country's security services, who hunted down and shot dead the man they identified as the ringleader, Abdelhamid Abaaoud, five days after the attacks.
Europol, the European Union's police agency, says it has been feeding information to the Belgian and French authorities but acknowledges that some member states are better at sharing information than others.
The focus of investigators over the past few years has been men and women who have grown up in Europe, have European passports and who travel to Syria to train and fight.
As the number of those fighters has increased, authorities have struggled to keep up. The French Interior Ministry estimated about 500 French nationals had traveled to Syria and almost 300 had returned. French authorities reckon up to 1,400 people need 24-hour surveillance. Yet France has only about the same number of officers to carry out the task, a tenth of those needed.
Some 350 people from Belgium have gone to Syria to fight - the highest per capita number in Europe. A Belgian government source said Belgium has a list of 400 people who are in Syria, have returned or are believed to be about to go there. There are another 400-500 people who authorities believe have radicalized. The number of people in the Belgian security services carrying out surveillance is believed to be considerably fewer than this.
The numbers partially explain why many of the attackers in Paris were well-known faces still at large.
The attacks killed 130 people at various locations, including the Bataclan concert hall where 89 concert-goers were gunned down or blown up. Others were killed outside the Stade de France sports stadium and in bars and restaurants around central Paris.
Seven assailants died during the attacks. Abaaoud was killed in a police raid north of Paris on Wednesday along with one other suicide attacker and a woman believed to be his cousin.
Dozens of people have also been detained, some with weapons and explosives, in raids since then.
Abaaoud himself had been well-known to authorities for several years. After a raid in January in the Belgian town of Verviers, police suspected the 28-year-old of plotting to kidnap a police officer and kill him.
In February, Abaaoud said in an interview with an Islamic State magazine that he had returned to Syria after the raid in Verviers. By this time, he knew he was being sought.
If it is true that he returned to Syria from Verviers, Abaaoud made his way back into Europe at some point after January. 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.
