The man who purportedly beheaded American journalist James Foley in a video clip uploaded to social media on Wednesday was almost certainly British.
Unconfirmed reports say the man with the English accent is from London and answers to the name John.
As the FBI and MI5 rush to identify him, his video appearance is a stark reminder to Washington and London about the number of foreign fighters in Syria and Iraq.
According to British government figures, as many as 500 Britons have taken part in the Jihad over the past two years. Hundreds more come from the US, France, Belgium, Germany and the Netherlands.
Several sources suggest as many as 20,000 fighters from nearly 70 countries have pledged their allegiance to Islamic State.
Recruited mainly through the Internet, there is no one clear profile of those who decide to sign up.
Experts say many of them have difficulties in forging an identity in a globalised world, are Muslims marked by unfolding events or criminals who have become radicalised in prison.
Free to move around with a Western passport, authorities are unclear what happens to them when they return from the Middle East.
The fear is that they can reproduce many of the tactics learned in training camps.
French Foreign Minister Laurent Fabius warns that Islamic State is a threat not just to the Middle East but also to western Europe and the US.
“We believe that this terrorist group probably constitutes a different danger and is different from the others.”
The involvement of foreigners builds a stronger case for some European countries to send military supplies to Kurdish fighters in northern Iraq.
German Foreign Minister Frank Walter Steinmeier confirmed on Wednesday that the country would send military aid, breaking a post-Second World War pledge not to arm foreign groups.
“We can imagine providing further top-range equipment, including weapons. Great Britain, Italy and France-have decided to send arms and we are prepared to do so too,” he said.
The aim seems to be about preventing Western jihadists from thinking about joining up.
British Prime Minister David Cameron says states must also toughen laws so that extremists can be stopped from travelling.
“What we must do is re-double efforts to stop all our people going, to take away the passports of those contemplating travel, to arrest and prosecute those who share in this extremism and violence, to take extremist material off the internet and do everything we can to keep our people safe.”
But will these measures be enough to stop westerners being brainwashed into joining Islamic State and then bringing their deadly jihad closer to home?