The video of a masked jihadist threatening to behead the American reporter James Foley shocked the world within minutes of emerging on social media.

With few clues to identify the Isis killer making demands on the US government, British intelligence analysts focused on his London accent, the veins on his knife-wielding left hand, and his posture — and “within hours” identified him as Mohammed Emwazi.

The voice in the video was disguised. But police and security services had been investigating Emwazi for three years before he slipped off their radar in 2012 and fled the UK for Syria. When sound analysts got to work, they made a match on the video, which was released on August 19, 2014.

The veins, when compared with those on