(Sirens)

DAVID LIPSON, REPORTER: Hours before a 28-year-old Australian man calmly walked into this mosque and murdered 51 people, he posted a 74-page manifesto laying out his motivation on a website called 8chan.

Slogans and catchphrases from the same website were scrawled on his rifle.

FRED BRENNAN, 8CHAN FOUNDER: It's clear that the Christchurch shooter spent a lot of time on 8chan - that probably emboldened him to commit his atrocity.

DAVID LIPSON: The gunman wasn't the last to boast of his crimes on 8chan, which became a go-to for mass killers and Neo-Nazis.

It was created by this man, Fred Brennan.

FRED BRENNAN: Nice to meet you.

DAVID LIPSON: Great to meet you. Hello, how are you?

Brennan, who has brittle bone disease, now lives in Manila, where 8chan was run but he grew up in the United States.

As a teenager, he found a refuge in a primitive message board site called 4chan, where users around the world post comments and share images, memes and pornography.

FRED BRENNAN: If we're honest, I was a teenage boy. I was drawn mostly because it was a site full of pornography.

I was very isolated in real life. I didn't have very many real-life friends.

I've known people that wake up in the morning, hop on one of these chan sites, and they're on it until night and it's just eight, nine, 12 hours a day. More, for some of the ones that are really into it.

DAVID LIPSON: By the time he was 19, Fred was disillusioned with the site he'd spent his teenage years immersed in and started to imagine an alternative.

It came to him while he was tripping on magic mushrooms - an anonymous message-board site where the users were in charge.

Can you describe what 8chan is in your own words?

FRED BRENNAN: 8chan is just a very simple kind of site. When you go on the board, you see the latest stuff, no matter what it is.

It's very much like a Facebook group except everyone's anonymous. There's no names.

DAVID LIPSON: What kind of people are attracted to that sort of world?

FRED BRENNAN: It's predominantly male, almost all white.

I mean, there are some minorities that are using it, but that, it's just not the norm.

DAVID LIPSON: But it wasn't long before Brennan's creation turned ugly.

In August 2014, hordes of male video gamers relentlessly targeted women in the gaming industry with threats of violence and rape.

8chan became their natural home because, there, the users policed their own behaviour.

FRED BRENNAN: Before all this happened, there was a narrative and not only in the community, but in the media, that, "Oh, it's just the cost of free speech and this is the cost of true free speech and if we all just talked, the best ideas will fall out."

Well, 8chan had been around for five years, and I never saw any good idea fall out, sorry.

DAVID LIPSON: With its new anonymous users, 8chan soon morphed into a forum for white nationalists, Neo-Nazis, and mass shooters.

Burned out, Brennan eventually quit the site in 2016 and, by the beginning of this year, he'd cut all ties with the new owners.

Then came Christchurch.

FRED BRENNAN: I expected them to see, now that 50 people had died, "We need to do something. We need to take this seriously."

And that just did not seem happen. They didn't do anything.

DAVID LIPSON: In the coming months, two more mass shootings would be connected to 8chan - the synagogue shooting in Poway, California, and the department-store massacre in El Paso, Texas.

FRED BRENNAN: There is a cohort of users on 8chan that celebrate mass shootings.

A lot of people will try to rationalise it and say "They're just trolling." No, they're not.

DAVID LIPSON: Fred Brennan decided he had to take a stand, and started blitzing the media, calling for the site he created to be taken down.

Within a couple of days it was off the air - not killed off by some government regulation, but by the companies that provide the digital framework and protection of the internet itself.

They dropped 8chan as a client and running the site after that became impossible.

Australia is leading efforts to clamp down on other websites. The Prime Minister has called on the G20 to band together to remove extremist material online.

SCOTT MORRISON, PRIME MINISTER: It's very important that we ensure that the rules that apply in the physical world apply in the digital world as well.

DAVID LIPSON: What's your view on that?

FRED BRENNAN: I feel like the, the time we're living in is gonna be limited.

You know, where every country is just allowing you to access sites that don't follow their local laws, there's really no benefit to that for Australia.

DAVID LIPSON: Once the ultimate champion of unbridled free speech online, Fred Brennan's view now couldn't be more different.

FRED BRENNAN: I guess the only thing that could happen is something that's extremely unpopular right now and it would be some kind of United Nations Security Council for the internet, say a United Nations Internet Council.

I mean, we've got to try something, don't we?

This era that we're in where the government power stops as soon as you plug in a network cable, I don't know that that's going to continue.