Militant extremists are thought to have exploited a deal agreed by American-backed Kurdish soldiers and Arab forces to escape war-torn Syria.
Initially it was reported that around heavily armed 4,000 ISIS fighters and their families were evacuated by a group of truckers, who had believed they would be helping civilians.
At the time, Colonel Ryan Dillon, spokesman for the US-led coalition fighting ISIS, told the BBC that US knowingly accepted the deal.
“We didn’t want anyone to leave,” Dillon said. “But this goes to the heart of our strategy, ‘by, with and through’ local leaders on the ground.
“It comes down to Syrians – they are the ones fighting and dying, they get to make the decisions regarding operations.”
However, according to The Times, many of these fighters may be heading west, instead of staying in Syria and Iraq, reporting that dozens have since been caught trying to cross the border into Turkey.
In a report in the newspaper, one of those arrested, Saddam al-Hamadi, 26, said that hundreds of Isis fighters had exploited smuggling routes into the country.
“I took advantage of that movement during the evacuation deal to get to Turkey. Lots of people were doing it, around half of them fighters and half of them civilians.
“It was an easy route to cross. Even if the YPG [the western-backed Syrian Kurdish militia] catch you, you will be held for ten or 15 days and then released,” he said.
“The evacuation was such big news that everyone heard of it,” he added.
“I got on the road at the same time as the agreement. I crossed some farms and villages and got to Manbij and then al-Rai. The YPG didn’t ask anything.”
A Turkish official said that there had been a significant increase in the number of people caught.
The Syrian Democratic Forces, a Kurdish and Arab alliance that fights Isis, allowed members of the group to leave Raqqa in order to salvage what was left of the city.
At the time of the initial report, a young ISIS fighter from France said that many fighters would return to Europe.
“There are some French brothers from our group who left for France to carry out attacks in what would be called a ‘day of reckoning’,” he told the BBC.
According to The Times, a source close to UK intelligence agencies said that “the UK is genuinely worried about the return of Isis fighters.”
“There are also concerns that intelligence information may not be shared as quickly as it was because of the bad blood over Brexit, and the EU saying the UK will have to leave Europol and the intelligence-exchange mechanism.”
Think tank The Global Strategy Network estimated that 400 Isis militants from the UK remain unaccounted for.
Richard Barrett, the author of the report, told The Times: “Returnees who were still fighting as Raqqa fell are likely to be far more committed, more violent and more radicalised than the current crop of returnees.
“I am quite sure that the UK authorities would be extremely keen to know who they are or were, where they are, where they intend to go, and what they intend to do. They would also want to know about their associates, as this group is unlikely to distinguish much between nationalities, having forged close bonds under intense pressure.”