Counter-terrorism police are taking down around 1,000 sites a week believed to be contributing to radicalising people to extremist views, MPs have been told - though the people behind them remain mostly at large.
Metropolitan Police chief Sir Bernard Hogan-Howe said officers had several lines of inquiry about people trying to radicalise people towards extremist views, and who may have been involved in recruiting three teenage girls who ran away to join militants in Syria.
He said efforts were being made to prevent radicalisation in prisons and communities, but said the internet was an emerging and growing problem.
He said officers were "sometimes" able to track down those behind the sites, but often they were based overseas, which made things more difficult.
New footage appears to show three missing London schoolgirls in Turkey during what is believed to be their journey to Syria.
The families of three schoolgirls who fled to Syria to join IS have questioned claims they funded their journey by selling family jewellery.
Relatives of London schoolgirls thought to be in Syria tell ITV News police failed to warn them the girls were at risk of radicalisation.?