From the weekend shootings in the United States to the mosque attacks in New Zealand - and many more besides.
They are all linked to websites which allow hate and radical speech.
The suspects thought to have shared their manifestos before carrying out their murderous attacks.
They didn't 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 - be anonymous and 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?
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.
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