How was Hayat Boumeddiene, the fugitive partner of one of the Paris attackers, able to cross through Turkey into Syria so easily?
Turkey may have stepped up efforts to improve security on its border with Syria, but the area remains a major thoroughfare for jihadists and their sympathisers seeking to enter the country from Europe.
The flight of Boumeddiene, 26, the partner of Jewish supermarket assailant Amedy Coulibaly, has revived questions over the porousness of Turkey's borders, and Ankara's political will to confront the threat posed by IS jihadists.
Boumeddiene entered Turkey on January 2 at an Istanbul airport, passing through immigration -- then the entire country -- without being stopped.
Security footage broadcast by Turkish media shows her passport being stamped at an Istanbul airport, then the young woman being waved through with an unidentified male companion.
A week later, on January 8, she confirmed having crossed into Syria.
"The Syrian border is still open, whatever they say. Provided you're spending a few dollars, everyone can cross it in both directions without any problem," a Western diplomat told AFP on condition of anonymity.
Turkey's border with Syria is by far its longest, and Ankara officials acknowledge is not easy to control, comparing it to the US border with Mexico.
Foreign Minister Mevlut Cavusoglu has said Turkey is taking "extreme measures" to ensure security along the restive border, but could never make it completely hermetic.
"A passage (into Syria) can always be found," he said.
Prime Minister Ahmet Davutoglu implied it was unfair to blame Turkey over a border that would never be fully controllable: "Is it Turkey's fault to have a border with Syria?" he asked.
'WE CAN'T FIGHT ALONE'
When France launched a hunt for Boumeddiene, she was already off Turkish authorities' radar. They confirmed her arrival from Madrid at an Istanbul airport only after Paris provided a tip-off on her whereabouts.
Despite that, Turkey says intelligence-sharing with Europe has increased in recent months and is "above normal", particularly with France. But it said that partnership must be further strengthened to fight a threat which does not concern only Ankara.
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"This is a problem that we cannot fight alone," a Turkish government official said, while also blaming Europe for its failure to fully integrate Muslims -- some of whom end up in Syria to take up arms.
"How can an individual born and raised in London, Paris or Madrid remain outside integration policies and eventually join IS and behead others?
"There are people in IS ranks not just from Turkey, but from more than 80 countries," he told AFP.
Turkey has drawn the wrath of the West for not doing enough to stem the flow of foreign fighters seeking to join IS, which has captured chunks of territory in Syria and Iraq right up to the Turkish border.
One of the criticisms is based on Ankara's tendency to see the regime in Damascus as the number one enemy, rather than IS -- which shares Turkey's goal of bringing down the Assad regime.
The protracted war and Assad's ability to remain firmly in power, however, have turned Ankara's calculations upside down, with the emergence of IS as a potential threat to Turkey.
"Turks believe that IS won't target them, or does not constitute a direct threat because they are Sunni (Muslims) like them, and they have a common enemy -- the Syrian regime," said the Western diplomat.
"They are sadly mistaken."
Officials say up to 700 Turkish nationals have joined radicals' ranks, and raise alarms over a threat to Turkey's national security with potential attacks when those recruits return home.
"Turkey has become a centre harbouring sleeper cells of terrorist organisations," opposition leader Kemal Kilicdaroglu said.
IS recruitment activities in Turkey are an "issue of radicalisation" which has moved deeper into Turkish society, said Nihat Ali Ozcan, security expert at the Ankara-based TEPAV think tank.
"Turkey not only has the problem of IS, but also PKK recruitment," he told AFP, referring to insurgents with the separatist Kurdish rebels.
Turkish authorities have also been rattled by a deadly suicide bombing earlier in January carried about by a Muslim female bomber from Dagestan -- who was reportedly the widow of an IS jihadist.