Last week’s deadly bombing of Istanbul’s international airport may have been blowback for Turkey’s move to try to stem the flow of fighters.

ISIS group extremists suspected in the blast could have been retaliating not only for the tightening of the frontier but also for a Turkish-backed offensive by Syrian rebels in northern Syria that threatens to recapture the last stretch of the border that ISIS still holds, observers say.

Turkey strongly denies ever intentionally allowing ISIS militants into Syria and insists it did its best to stop them. But for years the porous, 566-mile-long (911-kilometre) border has been vital for the extremist group, particularly at the time it was most intensely recruiting foreign fighters to build the so-called “caliphate” it declared across much of Syria and Iraq in the summer of 2014.

By the end of 2014, ISIS had 20,000-31,500 fighters, around half of them foreigners, according to CIA estimates. A large portion of those foreigners are believed to have entered through Turkey.

The Associated Press analyzed 4,037 “entry documents” logged by the ISIS group for its fighters entering from Turkey into Syria between September 2013 and December 2014. The documents were leaked to the Syrian opposition news site Zaman al-Wasl, which provided them to the AP. They alone would make up between 25 to 40 percent of the estimated total of foreign recruits, and they likely do not represent all fighters that entered through Turkey during that period.

The ease with which militants crossed into Syria from Turkey gave many observers the impression that Turkey’s government was at a minimum looking the other way as it prioritized support for Syrian rebels trying to oust Syrian President Bashar Assad.

Turkey’s relatively open border also benefited Syrian rebels, including ones backed by the United States, which used Turkish territory as a crucial rear base and supply route.

It was a life-saving escape route for some 2.75 million refugees who fled into Turkey and an avenue for humanitarian aid to opposition-held areas of Syria. But a recent interview with a Syrian smuggler appears to indicate that ISIS fighters too are moving freely across the border at various points. “Generally, Daesh [ISIS] members would not come and go as Daesh. They would come and go with civilians. So if you saw them, you’d think they were civilians, they didn’t look like fighters. They were mixed with civilians. The borders were very easy, there was no controls,” said the man, who spoke on condition of anonymity, for fear of arrest or violence from the militants.

A Turkish official, who spoke on condition of anonymity in line with government protocol, rejected the claim Ankara ever knowingly allowed terrorist fighters to cross into Syria.

He pointed out that Turkey arrested thousands of foreign fighters and sent them back to their home countries. Turkey deported about 3,250 foreign fighters from 2011 to March 2016, according to the Foreign Ministry. This year, Turkey has detained 1,654 IS suspects, Interior Minister Efkan Ala said recently. Of those, 663 remain in custody, more than half of them foreigners.

Turkish security forces have been busy rounding up ISIS suspects in the wake of the June 28 triple suicide attack at Ataturk Airport, which killed at least 44 people and wounded more than 230.

The Turkish official told the AP the main problem is that countries, including many European Union states, refused to share information with Turkey about terrorist suspects, who often crossed numerous borders before reaching Turkey.

He said requests for intelligence sharing about these suspects were not taken seriously until late 2014. He said some governments may have even allowed suspects to depart for Syria in the hopes that they’d be dealt with abroad.

The documents obtained by the AP were compiled by the ISIS “border authority” for fighters entering from Turkey.

Of the 4,037 entry documents, around 3,900 list the entry points.

They show 19 different areas used as crossings, with the majority crossing through three Syrian areas of Tal Abyad, Jarablus or Azaz. Those crossings correspond with Akcakale, Karkamis and Oncupinar on the Turkish side.

The documents, which also list the nationalities of the entering fighters, show they come from all over the world, including Europe, North Africa, the Middle East, the Caucasus region and South Asia.

German intelligence authorities have said that similar ISIS registration documents they have seen appear to be genuine.

The Syrian smuggler who works in the Syrian border area of al-Rai says that until November, the area, including the Jarablus crossing, was heavily trafficked by ISIS fighters. He says the group would bring in 30 to 40 people daily at the border at his village.

The ISIS entry documents list 190 fighters who crossed through al-Rai in 2014. The man also confirmed that smugglers like him are also finding it relatively easy to get across the border without being challenged. “Smuggling is taking place. We go in and out,” he said, speaking on condition of anonymity to discuss his illicit business. “When the Turkish government first tightened the border, it was very easy, very simple. Anyone could come and go, whether Daesh or civilian or whatever. There were no controls at all.”

Further east, the divided town of Tal Abyad, known on the Turkish side as Akcakale, was another important entry point for ISIS, opening to a road leading directly south to the “caliphate’s” de facto capital, Raqqa.

That route, however, was shut down when Kurdish led-forces took Tal Abyad in June 2015.

Partly under US and EU pressure, Turkey tightened its border controls last year.

Government authorities began overseeing construction of the wall, which will cover more than a third of the border when complete and include watchtowers and infrared thermal cameras.

Turkish guards began to push back Syrians too, trapping tens of thousands fleeing the conflict.

Since the spring, Turkey has been backing Syrian rebels pressing to take back the last 45-mile stretch of the border still controlled by the Islamic State (Daesh / ISIS) group, including Jarablus and the al-Rai area.

At the same time, US-backed fighters from the mainly Kurdish Syrian Democratic Forces have been waging their own offensive from the east trying to take the border area. Still, some Turks blame Ankara’s policies for the airport attack.

About 200 protesters shouted against the ruling Justice and Development Party last week, accusing it of supporting the ISIS group.

The recent deadly attack inside Istanbul’s international airport is believed to be the work of foreign fighters.

Turkish authorities say that the three suicide bombers who carried out Tuesday’s attack which killed 44 people and wounded more than 230, were from Russia and the Central Asian nations of Uzbekistan and Kyrgyzstan.

They have not provided further details on their identities, but Turkish President Recep Tayyip Erdogan has said the ISIS group is “most probably” behind the attack.

Turkey has blamed ISIS for several major bombings in the past year in Ankara and Istanbul.

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