When Benjamin Xu, a German national of Macedonian and Chinese descent, arrived in Istanbul two years ago, he had no problem hooking up with the handlers from the Islamic State in Iraq and Syria (ISIS) managing his transfer to neighbouring Syria.
Welcomed by a member of the extremist group who called himself simply Mohammed, he was taken to an Istanbul association that was officially billed as a charity organisation but was known in the neighbourhood as a relay station to transfer militants to Syria.
"It had a light green sign and looked like an aid association," Xu told Turkish investigators when he was arrested a year later.
Mr Tugay Bek, a lawyer who has been attending the ongoing trial against Xu, told The Straits Times it was an open secret in the Istanbul neighbourhood that the association was providing help for extremists. "Even the children on the street knew it, and they called the men the 'new fighters'."
Soon, other ISIS volunteers started arriving. Together, the volunteers travelled to the Turkish-Syrian border, where yet more helpers secured Xu's passage to a training camp in Syria.
"There are Chechens, Turks and Germans in the camp," Xu said, adding that the Chechen leaders of the training camp owned houses in Turkey's Hatay province just across the border.
Xu, 25, was arrested along with two other men after a deadly shoot-out on his way back from Syria into Turkey in April last year and has been in detention since.
His testimony to investigators, leaked to Turkish media recently, provides a glimpse of the ISIS network known to have funnelled thousands of foreign fighters and supporters from around the world to join them in Syria.
"Turkey is the most important transit point" for this highly organised effort, said Turkish opposition MP Ertugrul Kurkcu.
The flow of foreign fighters shows no sign of drying up. Five Dutch nationals were arrested as they tried to cross the border from Turkey to Syria last Saturday, the Turkish military said. Two weeks ago, 16 Indonesians were arrested.
Mr Kurkcu added that Ankara had stepped up countermeasures recently because of international pressure and because ISIS was becoming an internal threat to Turkey itself. "They take it a bit more seriously now," he told The Straits Times. "But it's a little late in the day."
The border crossing comes at the end of a process that starts very early - with a 50-page English-language booklet for potential new fighters planning to travel to Syria via Turkey.
The guidebook, Hijrah To The Islamic State, tells recruits to travel to Turkey, contact ISIS members in Syria via Twitter and ask them to come and pick them up. Its title is in keeping with other efforts by ISIS to endow its actions with religious symbolism as "hijrah" usually refers to the Prophet Muhammad's journey from Mecca to Medina to escape persecution.
"Lately, things have got harder at the Turkish border, so Islamic State members often meet new people in Turkey hotels and smuggle them across the border," the booklet says. It contains advice on what to pack - including the right sort of power plug adaptors for Syria - and separate tips for female militants.
The Guardian newspaper reported late last month that Britain's anti-terror police were trying to have the booklet removed from websites, but it is still available.
ISIS sometimes relies on Turkish smugglers to lead new arrivals into Syria. The extremist group controls several stretches of the 900km border on the Syrian side and can therefore receive new fighters once they make it over the line.
Turkey has rejected Western criticism that it is not doing enough to stem the flow of foreign fighters into Syria but admits there is a problem.
Foreign Minister Mevlut Cavusoglu said earlier this month that Turkey had blacklisted 12,519 suspected ISIS supporters to prevent them from entering Turkey and deported 1,154 foreign ISIS members.
But it is not always easy to identify potential fighters.
ISIS supporters from Western countries can easily travel to Turkey undetected because they do not need visas and can blend into the mass of millions of foreign tourists visiting the country every year.
Ankara also says home countries of ISIS supporters often fail to notify the Turkish authorities that suspects are on their way.
Western officials say the militants' networks might have benefited from Turkey's Syria policy in the early phase of the civil war that started in 2011.
They say Turkey initially supported radical Islamist groups, hoping they would speed up the ouster of President Bashar al-Assad.
Mr Francis Ricciardone, the former US ambassador to Ankara, said last year that the Turkish authorities were convinced they could work with radical militias in Syria, including groups linked to Al-Qaeda.
A Western official in Ankara, speaking on condition of anonymity, also said the Turkish government had been sure they could control the Islamist militants. "Now, all this is blowing up in their faces."
Critics say Ankara is reluctant to investigate the ISIS operation in Turkey thoroughly because ties between militants and members of the security apparatus could come to light. "There may be a certain tolerance by Turkish border guards towards ISIS," Mr Kurkcu said.
Xu's case also hints at a possible connection. He told investigators he decided to leave Syria after his father was killed by Syrian government forces.
He said that he managed to leave Syria with two other foreign militants and that a man called Heysem Topalca helped the trio to cross the border back into Turkey. News reports say Topalca is suspected of having ties to Turkish intelligence services.
