All five of the terrorist attacks that hit Britain last year had an "online component", the Home Secretary has said as she warned the internet is a "recurring theme" in atrocities.
In a speech in San Francisco, Amber Rudd highlighted difficulties in identifying potentially violent extremists in an era of "remote radicalisation".
She described various "stages of horror" in the wake of an attack - hearing what has happened, learning about the victims and discovering details of the perpetrators.
Ms Rudd said: "That's when you find out more about the terrorist.
"When you find out what it was that motivated them, what led them to carry out their unforgivable actions, and how they were able to do it.
"Increasingly we are finding a recurring theme. That theme is the internet.
"All of the five attacks on UK soil last year had an online component."
Ms Rudd cited the example of Salman Abedi, the terrorist who targeted a pop concert in Manchester in May, saying he had "learnt how to build a bomb from a YouTube video".
She emphasised that the internet itself is not the problem.
"But it does seem that those who commit terrorist murders on our streets are increasingly influenced by what they read and what they see online," Ms Rudd said.
"And this remote radicalisation can be hard to spot."
In the case of Daesh - also known as Islamic State - Ms Rudd said that once their material is online it spreads quickly.
"Our research shows the majority of the links to Daesh propaganda are shared within the first two hours of release and in that time, hundreds of messages promoting them are posted online across a number of different platforms," she said.
"We need to stop this sort of material getting onto the internet and helping to radicalise people."
Home Office analysis found IS supporters used more than 400 separate online platforms to pump out propaganda last year.
On Tuesday the Government unveiled new technology that aims to automatically detect terrorist content before it hits the web.