(SOFIA) - When Bulgarian police on January 1 arrested Frenchman Fritz-Joly Joachin, a self-confessed "old friend" of the Charlie Hebdo attackers, he was not the first suspected Islamist trying to cross the EU's south-eastern border into Turkey.
Indeed, Bulgarian Interior Minister Veselin Vuchkov says, "hundreds" of Europeans have travelled to Bulgaria to traverse the 275-kilometre (170-mile) border into Turkey, some then crossing into Syria to join Islamic State (IS) militants.
The most popular route for the several thousand European Union citizens believed to have joined the ranks of IS has been to fly to Turkey and then cross the border into Syria.
But with Turkish airports tightening up controls, going by land via Bulgaria, often by bus, has become more attractive, alarmed European officials say.
"With surveillance of arrivals at Istanbul airport being stepped up, this (Bulgarian) channel is being used more and more," a French anti-terrorism official in Paris said on condition of anonymity.
"Crossing the Bulgaria-Turkey border is relatively easy, particularly at night, when it's winter, it's snowing..."
Already in 2012 there were signs that extremists were using this route with the arrest in Bulgaria of Flavien Moreau, the first Frenchman to be convicted of fighting with jihadists, as he tried to return to Syria.
But the case of Joachin, 28, who admits knowing the Kouachi brothers -- who shot dead 12 people in an attack on the Paris offices of French satirical weekly Charlie Hebdo on January 7 -- showed how easy leaving Bulgaria can be.
Bulgarian police only arrested the Muslim convert of Haitian origin -- who denies being an extremist -- because of a European warrant based on claims by his wife that he was kidnapping their three-year-old son.
Only after the Paris attacks, six days after his arrest, did the French authorities issue a second warrant, this time for alleged membership of a "terrorist" organisation. He was extradited this week.
Moreover, his three travelling companions on the bus, who also allegedly knew the Kouachis, were allowed to continue their journey. And one of them, Cheikhou Diakhaby, should have set alarm bells ringing.
Diakhaby had spent seven years behind bars in Iraq after being captured while fighting against US forces in 2004 and was sent back to France in 2011. Turkish police managed to pick him up however on January 2 -- on the Syrian border.
- Tighter controls -
Bulgaria's focus until now has been trying to deal with people coming the other way, erecting a barrier topped by razor wire to stem the flow of thousands of refugees from Syria entering the country.
But lately, and in particular since the Paris attacks, Bulgarian authorities have turned their attention to people leaving the country -- and to EU citizens who may be returning after fighting alongside IS.
In recent weeks senior officials including US Secretary of State John Kerry, British counterpart Philip Hammond and EU counter-terrorism chief Gilles de Kerchove have visited Sofia with improved security cooperation high on the agenda.
Bus operators say that their vehicles are attracting more attention from customs officers at the main border crossing point. Passengers' passports are now routinely verified and cross-checked with the EU's Schengen database.
"A few months ago they started scanning all buses," said a manager at Metro, the biggest operator of buses from Sofia to Istanbul. Previously this only happened if customs officers were suspicious.
But the problem remains that the Bulgarian authorities say that they are not given enough information by other EU countries, and can only intercept people when they are alerts about them or an arrest warrant.
"Many hundreds of citizens of the EU, with absolutely valid documents and without any security alerts about them, can easily cross Bulgaria's territory," said Vuchkov, the interior minister.