A cat and mouse game is taking place every day on the internet. It is the online battle to deny space to extremist content.
On one side of the frontline are a team based in New Scotland Yard – the headquarters of the Metropolitan Police.
On the other are jihadists based in Syria, Iraq and elsewhere posting propaganda videos online – the most grisly of which show the beheading of hostages.
“We take stuff down, they put more stuff up,” explains one of the Scotland Yard team who spoke to the BBC but asked to remain anonymous for his own security.
He is passionate about the importance of his job even though it has its challenges.
“It can be quite draining having to continuously see graphic and horrible images,” he says.