The Syrian passport that was found near the body of an assailant in Friday night's terror assault in Paris entered the European Union through Greece last month, an official said.
A Greek minister said Saturday the man had crossed into the E.U. through the Greek island of Leros in October.
"We announce that the passport holder had passed from Leros on Oct. 3. where he was identified based on E.U. rules... We do not know if the passport was checked by other countries through which the holder likely passed," Citizen Protection Minister Nikos Toskas, who is in charge of police forces, said in a statement, which was confirmed by a Greek police source to NBC News.
"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."
A French official close to the investigation told NBC News that a Syrian passport was found on one of the attacker's bodies, but could not confirm that the attacker was the passport holder. A black market for Syrian passports is booming: Migrants looking to flee the nation's civil war sometimes turn to forgery to escape.
The coordinated attack across the French capital killed 129 people and injured at least 352 others, officials said.
The eight assailants attacked a concert hall, restaurants, and a soccer stadium. Seven blew themselves up, while the eighth was killed by police, authorities said.
ISIS claimed responsibility for the attacks on Saturday as French President Francois Hollande vowed a "merciless" response.
U.S. intelligence and law enforcement agencies have been concerned about the possibility of ISIS militants and other radicals infiltrating the flow of refugees making their way to Europe, a senior military intelligence official told NBC News.
Officials are particularly concerned about refugees using falsified documents and stolen blank passports. ISIS acquired a virtually unlimited supply of blank passports when it took over large swaths of Iraq and Syria in the summer of 2014, the official said.
American, NATO and E.U. officials, including France, held a number of meetings earlier this year to discuss the problem and come up with a tighter form of biometric sharing, which involves facial recognition, palm printing, retina scans and other technology, according to the official. Jordan and Iraq have also participated in the biometric sharing.
U.S. intelligence is also concerned about the infiltration of ISIS fighters in the boat lift between Libya and Italy, according to U.S. officials.