All terrorist attacks on Britain last year involved an “online component”, Amber Rudd has said as she warned that the internet is a “recurring theme” in atrocities.
The Home Secretary highlighted difficulties in identifying extremists in what she described as an era of “remote radicalisation”.
Speaking in San Francisco on Tuesday, Ms Rudd used Salman Abedi, the terrorist who targeted the Manchester Arena in May, as an example, saying he had "learnt how to build a bomb from a YouTube video".
She also 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."
But 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 rapidly.
"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.
