GETTY•REUTERS Greek officials said the passport of the man showed he was Syrian

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The shocking development suggests the killer may have been posing as a refugee before travelling across the continent to carry out the atrocity. Officials have also revealed a second man who carried out the mass murder was likely to have a passed through to the continent in a similar way. A Greek government official revealed the first terrorist, who died in the series of attacks on the French capital, held the document of a man who had passed into the southern European country, probably by refugee boat, last month. The passport was found near the body of the terrorist. Greece said the holder of the document arrived in mainland Europe from the island of Leros, suggesting he had made his way there by sea. It has been revealed the man was one of the suicide bombers at the Stade de France. He was not known to the French authorities. Citizen Protection Minister Nikos Toskas, in charge of police forces in Greece, said: "On the case of the Syrian passport found at the scene of the terrorist attack, we announce that the passport holder had passed from Leros on October 3 where he was identified based on EU rules.

GETTY Huge passenger ferries transported migrants from islands like Leros to Athens

GETTY Tens of thousands of migrants arrived at Greek islands from outside of the EU by boat this year

the passport holder had passed from Leros on October 3 where he was identified based on EU rules Nikos Toskas

"We will continue the painstaking and persistent effort to ensure the security of our country and Europe under difficult circumstances, insisting on complete identification of those arriving. "We do not know if the passport was checked by other countries through which the holder likely passed." A Greek police force source said the passport owner was a young man who had arrived on a small vessel from Turkey with a group of 69 other refugees. He had his fingerprints taken by officials. Greek officials revealed a second terror suspect was also likely to have passed to an island via the same route.

They are currently investigating the development and are looking to match the remains of the attackers with the passports. Police have refused to name the Syrian passport holder, saying France wanted to keep it from the public. But they say the man was believed to have left Greece to go to Macedonia Tony Smith, former head of UK Border Force said it was "very worrying". The migration expert said: "We expected either lone wolves or home grown terrorists radicalised before travelling to Syria and then coming back. "The complexities of this group, that they were fingerprinted, suggests that there have been some breaches of external barriers in Europe." The latest development comes as Belgian police have arrested a number of people in Brussels suspected of being linked to the attacks, according to the country's justice minister Koen Geens.

GETTY France has declared a three-day period of mourning following the killings

French media reported three of the attackers were from the Molenbeek area of Belgium, where police raids were carried out today. An eye-witness said one of the cars used by the terrorists had a Belgian numberplate. Another car is also believed to have a Belgian numberplate, French sources said. Both vehicles were black. Another of the terrorists, who carried out killings at the Bataclan theatre, is said to have been from the Paris suburb of Courcoronnes, 20 miles south of the French capital. He was understood to have Islamic links and was known to the French authorities. The French prosecutor said the 30-year-old had been arrested for eight minor offences in the country and was on the radicalisation watch list since 2010. Courcouronnes is the same area as Amedy Coulibaly was from - the killer who was involved in January's Charlie Hebdo shootings before murdering four hostages at a kosher supermarket nearby.

Coulibaly was then killed by police. Following the terror attacks on Paris last night, Islamic State (ISIS) members on Twitter claimed responsibility for the killings.