The Syrian passport was registered in October in Serbia and Croatia, two of the countries on the corridor crossing the Balkans.
The owner was allowed to proceed because he passed what is essentially the only test in place, he had no international arrest warrant against him, police in both states said today.
It was not clear whether the passport was real or fake, or whether it belonged to the suicide bomber. But trafficking in fake Syrian passports has increased as hundreds of thousands of people fleeing war and poverty try to get refugee status, the chief of the European Union border agency Frontex has said.
But, many people tell officials that they've lost their identity papers, and they can give false names and other information, including their country of origin.
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A large majority of migrants declare themselves as Syrians from the war-torn country, although they have no documents to prove it, Serbian police say.
Being Syrian refugees have a better chance of getting asylum in Germany than those classified as economic migrants from Afghanistan, Iraq or Pakistan.
"No one can know for certain where they come from, their true identity or if their documents are genuine," Serbian labor minister Aleksandar Vulin said.
"The Paris suspects have not been registered anywhere as terrorists, so Serbia could not have known that they represent a danger."
Frontex spokeswoman Ewa Moncure told The Associated Press that "among some 500,000 people, you will find some with false documents."
The holder of the passport found next to the bomber's body is registered as entering Greece on October 3, Greek officials said, adding that the passport owner entered the country through Leros, one of the eastern Aegean islands that tens of thousands have been using as a gateway into the European Union.