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.

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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 by Reuters 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.

FOCUS ON FIGHTERS RETURNING FROM SYRIA

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.

WARNINGS

Mostefai, the Bataclan suicide bomber, also traveled back and forth. Although he had eight convictions as a petty criminal, he had never been in prison, a place French authorities can watch for signs of radicalization.

Police say they suspected him of being in Syria between late 2013 and early 2014, before returning to France unnoticed.

In December of last year, Turkey contacted France about Mostefai. They raised an alarm again in June 2015 by letter.

There was no response from French authorities, according to a senior Turkish government official and a security source.

"It seemed there was a connection between this person and Daesh (Islamic State) and we reported it," the Turkish security source said. "We followed all international procedures. But they (the French) didn't display the same level of sensitivity."

French officials declined to comment on this, but say that coordination with Turkey over potential French jihadis has improved markedly in the past year.

Determining how dangerous a person is, and whether they might carry out an attack, is a key challenge for security services, experts say.

"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 Arnaud Danjean, a former intelligence officer and now a member of the European Parliament.

Bilal Hadfi, who blew himself up outside the Stade de France, was another of the suicide attackers under surveillance.

After visiting Syria in February, the 20-year-old French national, who was living in Belgium, returned to Europe by an unknown route and evaded police even though the Belgian Justice Ministry said microphones had been placed at the house where he was thought to be staying.

Then there's the case of Sami Amimour. French authorities had launched an official investigation into Amimour's possible terrorism-related activity in October 2012. Prosecutors suspected him of planning to join militants in Yemen.

Amimour was a bus driver who had been radicalized in a mosque near his hometown of Drancy, north of Paris. Because of the investigation, police had ordered Amimour to check in with them every week. As reported by Reuters on Nov 20, he missed four weekly checks in 2013. But it was only after nearly a month that the authorities put out an international arrest warrant.

By then Amimour was already in Syria. His tracks were picked up a year later, in December 2014, when his father gave an interview to French daily Le Monde describing how he had traveled to Syria but failed to convince his son to return.

THE MEN FROM THE BAR

Police are still looking for Salah Abdeslam, who is known to have survived the attacks.

Until six weeks before the attacks, Salah and his brother Brahim - one of the suicide bombers - 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 Abdeslam went to ground. Authorities say he was stopped on his way back to Belgium after the Paris attacks, but police waved him on. It is not clear what role he played on the night of the attacks and why he managed to survive.

Two men who were arrested later, Mohamed Amri, 27, and 21-year-old Hamza Attou, said they brought Abdeslam back to Brussels after receiving a call from him saying his car had broken down. Police checks meant they were pulled over three times, including a last check around 9 a.m. near Cambrai just short of the Belgian border.

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 traveled 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 man traveled with a second man, who also blew himself up near the Stade de France.

The pair may have reached Paris relatively easily because, at the height of the migration crisis in Europe this year, asylum seekers were rushed across some national borders without checks.

It is unclear whether the passport issued under the name of Ahmad al-Mohammad, a 25-year-old from the Syrian city of Idlib, was genuine or was stolen from a refugee. Whatever the truth, it has helped fuel right-wing criticism in Europe of the number of migrants allowed in this year.

By the time the two men were making their way up through the Balkans to western Europe, France had received more evidence an attack was imminent.

French former anti-terrorism judge Marc Trevidic says a French Islamist he questioned on his return from Syria in August said Islamic State had asked him to carry out an attack on a concert venue.

"The guy admitted that he was asked to hit a rock concert. We didn't know if it would be Bataclan or another, he didn't know the exact location that would be designated. But yes, that's what they asked him to do," Trevidic told Reuters.

Iraqi Foreign Minister Ibrahim al-Jaafari has also said that his country's intelligence services shared information indicating that France, as well as the United States and Iran, was being targeted for attack. He has not given details.

Germany's top prosecutor is also investigating allegations that an Algerian man detained at a refugee center in the western town of Arnsberg told Syrian refugees an attack was imminent in the French capital.

Europe is scrambling to respond to the attacks.

France declared a nationwide state of emergency which will now last three months. Police now have the power to conduct searches without obtaining judicial warrants and can hold anyone suspected of posing a threat to security under house arrest for 12 hours a day. Internet sites deemed to incite or advocate "acts of terrorism" can be blocked and public demonstrations banned.

Belgium has also announced a security crackdown, saying it will spend an extra 400 million euros ($430 million) on security and take measures such as stopping the sale of mobile phone cards to anonymous buyers. Police will be allowed to conduct night searches of homes and it is now easier to ban, convict or expel hate preachers.

Whether such measures will be enough is uncertain. Brussels is on high alert this weekend because of what authorities there called the "serious and imminent" threat of attack. In a video last week, Islamic State warned it would strike again.

"When a large operation is prepared, they are told to keep a low profile in the months before. As‎ they are no longer on police radars, it's like looking for a needle in a haystack," said Roland Jacquard, president of the Paris-based International Terrorism Observatory.

(Robert-Jan Bartunek reported from Brussels and Orhan Coskun reported from Turkey; additional reporting by Nick Tattersall in Turkey, Alastair Macdonald in Brussels, Silvia Aloisi in Athens; writing by Timothy Heritage; editing by Alessandra Galloni, Simon Robinson, Janet McBride)