The Serbian magazine Blic displays a Syrian passport found by police at the scene of one of the Paris attacks. The passport turns out to have been a fake. (Andrej Isakovic/AFP/Getty Images)

The mystery man arrived in Europe on a boat from Turkey that washed up on the azure shores of Greece on Oct. 3. He disembarked with 197 desperate migrants on the isle of Leros, where harried police processed the man whose Syrian passport named him as Ahmad Almohammad, a 25-year-old from Idlib.

He then followed the same trail into Western Europe trodden by hundred of thousands of asylum seekers escaping war and conflict in the Middle East, passing through Athens, then Macedonia and Serbia, according to Greek and Serbian officials.

Then the man disappeared — until Friday, when he reappeared, this time in the form of the disfigured body of a suicide bomber in Paris, outside the Stade de France.

The fingerprints on the body, French and Greek officials say, matched those of the man who arrived in Leros on Oct. 3. That Syrian passport, French authorities confirmed on Tuesday, was a fake.

What we know so far about who carried out the Paris attacks

“It is obvious now,” said Bernard Squarcini, the former chief of French intelligence. “Amongst the migrants, there are some terrorists.”

The case of the mystery bomber has now ignited a political culture war across both sides of the Atlantic — one that revolves around whether it is migration or fear of migration that poses the greater peril.

Some have seized on the case as evidence that migrants and militants can be indistinguishable. But others worry that needy migrants will become the victims of a new wave of security paranoia — prompting an unusual warning Tuesday from the United Nations to the European countries that have pledged to house asylum seekers.

In the case of the mystery man, the debate is complicated by the falsified passport, which means that no one knows his true name or nationality, or whether he was truly a migrant. His name was clearly not Ahmad al-Mohammad, and refu­gee advocates say the man may not have been Syrian.

There is no question that, on rare occasions, militants are slipping through border controls with migrants. Moroccan national Abdelmajid Touil, suspected of supplying weapons used in a March terrorist attack in Tunis that killed more than 20 tourists, had traveled to Italy a month earlier on a migrant boat from Libya. In May, he was arrested near Milan.

Now French officials are alarmed by the prospect that several of the assailants involved in Friday’s attacks — seven of whom are dead, with two suspects on the run — may have tapped the migrant routes. “We are looking very closely at their travels,” said a French official involved in the case who spoke on the condition of anonymity because of the sensitivity of the issue.

Authorities say as many as 20 people may have been involved in the plot to attack Paris. Here's what we know about them so far. (Gillian Brockell/The Washington Post)

Concern is particularly focused on overburdened European entry points, nowhere more than in bankrupt Greece. Many of the asylum seekers flooding Greece are arriving without passports. Theoretically, interpreters are supposed to assess their claims by asking certain questions — What is the capital of Syria? — and assessing their accents.

But many “fake” refugees are unquestionably getting through. In September, officials in Germany said almost a third of asylum seekers there claiming to be Syrian were not actually from Syria.

Fake Syrian passports are considered hot commodities on the black market across the narrow straits in Turkey, where a cottage industry has arisen of Moroccans, Tunisians, Algerians and Egyptians seeking to pose as Syrians. The reason: Syrians have an exceptionally high acceptance rate as refugees in countries such as Germany, which offers generous benefits and housing to those genuinely fleeing war and persecution.

In the wake of the Paris attacks, Marine Le Pen, head of the far-right National Front party, has called for an immediate halt to the influx of migrants into France, saying that “our fears and our warnings about the possible presence of jihadists among the migrants who join our country have unfortunately been realized.’’ In the United States, more than two dozen Republican governors have said they plan to try to block Syrian refugees from resettling in their states.

French officials on Tuesday issued a police bulletin with the mystery bomber’s image, appealing to the public for details on his identity. The few details that have emerged about his journey have raised red flags about the mechanisms now in place.

After landing in Greece on Oct. 3, the man claiming to be a Syrian named Ahmad al-Mohammad traveled to Athens, then followed the flow of refugees north, through Macedonia and Serbia, where he applied, along with hundreds of others, for a transit visa in the Serbian town of Presevo on Oct. 15.

The European agency meant to help guard the continent’s borders says it is dramatically overburdened and underfunded, despite pledges of boosted funds from the European Union. The agency, Frontex, said Tuesday it was in urgent need of additional officers to fingerprint and screen migrants. At the beginning of October, Frontex requested 775 more border guards but received only 320.

Emily Badger and Virgile Demoustier in Paris and Stephanie Kirchner in Berlin contributed to this report.

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