Jake Davison, the man suspected of killing five people in Plymouth had been active on social media platforms discussing the "incel" movement - young men describing themselves as "involuntarily celibate".
At least two previous mass killings, both in the United States have drawn attention to the online activities of these groups.
So what do we know about those who participate in these forums?
Failure and frustration
"Obviously I don't have much hope of attracting a woman in the first place."
Jack Peterson is one of the thousands of young men who visit forums on Reddit and other incel websites.
"I've had a couple of negative experiences in relationships and so that has made me feel, like you know... it's hard to move on from my past and start a new relationship," he says. "I've had women who have done pretty bad things to me."
These overall themes of failure and frustration - along with anger and hate - are common on incel forums. I spoke to several young men who call themselves incel. They're in their teens or early 20s and many have experienced rejection or negative encounters with women.
They go to message boards out of loneliness. What they find is a group of angry men pushing the idea that they have lost the genetic lottery, and there's almost nothing they can do about it.
Jack has a YouTube channel and a podcast. He is one of very few incels prepared to speak to the media after the Toronto attacks in 2018.
Others agreed to speak if I changed their names. They're embarrassed because they're unsuccessful at relationships - but many are also hiding extreme views.
One incel from the UK - I'll call him Liam - has been active on forums from a young age. He's unemployed, and lives at home with his family.
He admits: "I've got this sort of misogynistic view which is not helping me in life, generally."
I ask him directly: does he hate women?
"In some ways, yes," he stutters. "I try not to, but I find myself like - I'll just be talking and I'll say things that I shouldn't really say, just because I've been looking at forums."
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