Reports have emerged that the executioner of US journalist James Foley is an educated, devout believer in radical Islamic teachings from Britain. He is said to be the leader of a group of British Islamic extremists based in Syria. Together with two other British-born militants, the trio have been nicknamed "The Beatles" because of their nationality.
Who are these young men who have gone to fight in another country's war? What are their goals and do they pose a threat to Britain's security?
Here are 8 things about them.
1. How many British Islamic extremists are there?
British intellgence believe there are 400-500 British nationals who have travelled to Iraq and Syria to fight among the ranks of militants from the Islamic State in Iraq and Syria (ISIS), also known as the Islamic State (IS).
2. Who are they?
Nasser Muthanna (centre), 20, from Cardiff, appeared in an online recruitment video in June 2014, urging Muslims to join the wars in Syria and Iraq. -- FILE PHOTO: REUTERS
Intelligence sources say most British jihadists are in their 20s, university-educated and Muslims of British Pakistani origin.
According to BBC, up to 20 people from the Sudanese community and several Syrians from Britain have also gone to fight in Syria.
One British jihadist, who calls himself Abu Muhadjar, says he had a good upbringing.
"I grew up in fairly nice area. I come from a decent family, close-knit family, well educated. Everyone in my family is a university graduate. I'd consider it a middle class family.
"My family do know where I am and what I'm doing. Just like any mother and father, of course they're going to be worried about their son. I'd be lying to say it's not dangerous here and no-one wants to bury their own children."
Another, 20-year-old Nasser Muthanna, from Cardiff, appeared in an online recruitment video in June 2014, urging others to join him.
Nasser, who had received four university offers to study medicine, appeared in the YouTube video dressed in a white turban, using the name Abu Muthanna al-Yemen and was flanked by five other men, three of whom appear to be British.
3. Why did they go to Syria?
Some experts believe that British Sunni Muslims initially went to Syria because of an "existential threat" to their faith from Shiite Muslims. The huge numbers of Sunni Muslims within the IS ranks have helped expand IS in size as well as territory and influence.
Some British members of the jihadi group have been keen to explain their motivations in their own words.
In a 13-minute video released by IS, entitled "There Is No Life Without Jihad", three Britons describe their motivation for travelling to Iraq and Syria in a bid to persuade others to swop a comfortable Western life for a religious war.
In the video, Abu Bara al-Hindi tells viewers: "The cure for depression is jihad... Feel the honour we are feeling, feel the happiness we are feeling."
Another Briton, still in the UK, told the BBC that he feels "obliged" to now go to Iraq or Syria because "God has commanded for the Muslims to go and fight jihad". He said to die as a martyr is "the promise of paradise".
Many of the jihadists say they are involved in aid work as well as fighting.
Explaining why he was in Syria, British jihadist Abu Muhadjar said: "There are many reasons made me leave my life and come here. The first is religious reasons - due to the fact that it's upon every single Muslim to protect Muslim lands and blood of Muslims if it's been transgressed upon.
"Second is humanitarian reasons - alongside of my fighting I tend to do aid work as well."
Qari Asim, imam at the Makkah mosque in Leeds, told BBC News the risk of young British Muslims joining the fighting was "getting greater because of our... involvement in Iraq".
4. Why Syria?
Shiraz Maher of the Centre for the Study of Radicalisation at King's College London says Syria has become a magnet for young Muslims across the world eager to take part in jihad - a holy war, or struggle to defend Islam.
"At the moment Syria seems to dominate the global jihadist mind, it is the premier location in the world to go and fight jihad today.
"One of the interesting things we've seen is that other jihadist groups, such as al-Qaeda in the Islamic Maghreb and (al-Shabab) in Somalia have issued a communique saying, 'Listen, don't forget about us, we also need foreign fighters, don't all go to Syria.'"
With Turkey only a budget flight away, it is cheaper for jihadists to get there and cross to Syria to fight than it is to get to Afghanistan or Somalia.
5. How are they recruited?
While some have gone under their own steam, the BBC said there are organised networks to get young men to go to Syria.
Salah al Bander, who runs the Sudanese Diaspora and Islamism Project, says extremists recruit these young men through prayer rooms attached to mosques and then arrange their travel.
"These kids don't have the means to buy a plane ticket. There is a kind of railroad between British and Western cities and linking points in Jordan, Lebanon and Turkey to bring these individuals inside Syria to go and join specific organisations."
But British jihadist Abu Muhadjar says the decision to go to Syria was his alone. "There's no recruitment process to come here. It's an individual decision that I took.
"Upon my arrival I found some like-minded brothers, I came across in the same manner. Their goals are the same so they're the ones that I was attracted towards the most and I joined them so we can fight together."
6. What's their role in ISIS?
Reports have emerged that the executioner of US journalist James Foley is an educated, devout believer in radical Islamic teachings from Britain. -- PHOTO: REUTERS
After the video of James Foley's execution was released, some analysts say the British executioner was used for propaganda purposes against the West and that in reality, British fighters are often given second-tier roles such as guards due to a lack of trust and their inability to speak Arabic.
But Shiraz Maher, from King's College London's International Centre for the Study of Radicalisation, said British IS recruits were "not there to take a back seat role" and many were becoming "hardened killers".
He told the Today programme that British fighters were "operating as suicide bombers; we have seen them operating as executioners.
"Unfortunately they are among some of the most vicious and vociferous fighters who are out there."
7. Are they a threat to Britain?
Britain's intelligence agency MI5 says thousands of Islamist extremists see the British public as a legitimate target for attacks.
Foreign Secretary Philip Hammond also warned there was a risk of extremists returning home to continue their terror.
"If the Islamic State, so called, becomes established in an area of Syria and Iraq, it will undoubtedly use it for a base, for launching attacks on the West.
"Equally if it gets pushed back, some will return to their countries, not just the UK, all European countries, Australia, the US, other Arab countries. We will see these people going back and potentially carrying on their fight in our homelands."
"We are absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with IS and other organisations."
But a British jihadist, who calls himself Abu Islam, says Britain has nothing to worry from him.
"For me personally I was born and raised there, that's my home. If I wanted to do something in UK I wouldn't have come here. If I did want to do something in UK I would have already done it by now, but I'm here."
Another British jihadist Abu Muhadjar also denies they will be a threat.
"That's slightly surreal to go back to UK and start a jihad there. I do understand their concern but you cannot paint everyone with the same brush.
"As to the global jihad - I couldn't tell you if I'm going to be alive tomorrow let alone future plans."
8. What is the British government doing?
British authorities have launched a hunt for the executioner of James Foley as Mr Hammond said British intelligence services would work closely with the United States to try to identify the man and a specialist counter-terrorism police unit launched an investigation into the contents of the video.
British authorities have withdrawn 23 passports this year and made 69 arrests in a bid to stop the flow of fighters travelling to Iraq and Syria.
A Home Office spokesman said: "The government will take all necessary steps to protect the public.
"Citizenship is a privilege, not a right, and the Home Secretary will remove British citizenship from individuals where she feels it is conducive to the public good to do so."
British police have been told to ban the flying of the ISIS flag in Britain and arrest anyone peddling recruitment literature for the group.
Sources: BBC, Independent, International Business Times