The suspects are believed to have shared their manifestos before carrying out their attacks.
They did not use browsers we all log in to every day – but a deeper, darker layer of the internet where you can hide your identity and location, as well as stay anonymous and be difficult to track.
The subterfuge raises questions about the responsibility of tech companies and websites to control what can be posted.
Governments are also exploring how to regulate the internet.
So Is there a correct balance between censoring violent content and maintaining freedom of speech?
Presenter: Hashem Ahelbarra.
Brian Hughes – professor at the American University in Washington, DC; fellow at The Centre for Analysis of the Radical Right where he studies extremism and digital technology
Caroline Sinders – online harassment researcher; fellow at Harvard Kennedy School and Mozilla Foundation
Ryan Broderick – senior reporter for BuzzFeed News covering online platforms and web culture.