UK nationals who trained and fought in Syria before returning home may pose a security risk, UK immigration minister James Brokenshire has admitted in a BBC interview.
Brokenshire told Radio 4's The World This Weekend: "A growing proportion of the security services' work is linked to Syria in some way. This is a big problem that the security services and the police are actively focused on."
Whitehall security sources believe most of the fighters, some of whom travel to Syria by taking EasyJet flights to Turkey and crossing the border, spent time in training camps affiliated to al-Qaida while in the Middle East. Some are veterans of the war in Afghanistan.
Since civil war erupted in Syria in 2011, around 400 UK extremists have travelled to Syria. Around 20 have died in the country, including Abdul Waheed Majeed, a 41-year-old from Martyr's Avenue, Crawley, who is suspected of being the first UK suicide bomber to die in Syria. Majeed was filmed looking relaxed moments before he carried out the attack on Aleppo prison, joking that he couldn't speak Arabic because his tongue had a "knot in it".
Around 100 UK fighters are thought to remain in Syria and 250 have returned home, though not all are thought to pose a threat.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said he was concerned that returning fighters may be more radicalised than before they left: "They may be injured or killed, but our biggest worry is when they return they are radicalised, they are militarised, they may have a network of people who train them to use weapons."
Last year a cell of returning jihadists was found to be planning a Mumbai-style attack in London.
Now the UK's most senior anti-terrorism police chief Commander Richard Walton has also voiced his concerns: "All our experience around Afghanistan is that if British Muslims get trained in camps they become a very real threat because the al-Qaida narrative extends to Western attack. It's almost inevitable."
Co-founder of the Quilliam Foundation Maajid Nawaz says the danger of young people travelling to Syria is even more serious than was the case in Iraq or Afghanistan, and that even those who initially go purely with humanitarian motives may become radicalised.