Two British nationals arrested by counter-terrorism police at Heathrow on Tuesday night are being questioned in connection with the abduction and attempted murder of a Sunday Times photographer in Syria during the summer.
The man and woman, both 26, were detained at 8.30pm after their return flight from Egypt on suspicion of the commission, preparation or instigation of acts of terrorism. Police were also searching two east London properties as part of the investigation.
Scotland Yard confirmed that one line of police inquiry was a possible connection with the capture and detention of a war photographer, John Cantlie, in August.
After being captured, Cantlie and a colleague were shot trying to escape from an Islamist training camp in north-west Syria. They were later treated by a British-born trainee doctor who said he was on leave from his hospital job in London.
After his subsequent release by members of the Free Syria Army, Cantlie described how his abductors, who included several Muslims from Britain, had threatened him with execution.
Responding to the growing presence of foreign fighters in Syria, the US national director of intelligence, James Clapper, said in February that those attacks "bore the earmarks" of the jihadists in neighbouring Iraq. Last month a UN panel warned that the number of foreign fighters in the conflict was growing – a development which it said could radicalise the rebellion against Bashar al-Assad's rule.
On Wednesday morning the foreign secretary, William Hague, warned against travelling to Syria to take part in the fight to depose Assad. Speaking on BBC Radio 4's Today programme after the arrests, he said the government was aware that British nationals were joining the battle for control of Syria.
"We also advise all British nationals to leave Syria, not to go to Syria. We don't supply anything that could contribute to lethal action inside Syria and we don't want individuals to do that either."
The Syrian government news service Sana siezed on the foreign secretary's comments as proof of outside involvement in the conflict.
"Hague's implicit confessions came in line with the divulging of the conspiracy hatched against Syria," the report read.
The agency said Hague's comments were in line with other, "international media reports which revealed the involvement of the US and some western countries in sending terrorists to Syria and providing them with military and financial support by some Gulf countries with the aim of toppling the Syrian state and spreading chaos".The Quilliam Foundation, a London-based thinktank studying extremism, estimated that there were 1,200-1,500 foreign fighters across Syria.
Maajid Nawaz, of the Quilliam Foundation, said the deepening of the Syrian crisis had globalised the conflict.
That British fighters had arrived in Syria, he said, had flagged up two issues.
"Although the UK has been relatively quiet [in terms of attacks], radicalisation in the UK hasn't been, because to get a group of Brits out to Syria so quickly, who are willing to kill their own fellow Britons, demonstrates that there was something already happening before the Syria uprising; there is an ideological mindset that they [already] adhere to.
"And that's something we've been talking about for a long time; even though there have been no bombs going off in the UK, the fact there is this extremist presence on the ground in the UK will once again become a threat."