After the New Zealand massacre was broadcast live on Facebook, it quickly went viral on various social media platforms.

Companies including Facebook, Twitter and YouTube scrambled to take it down, but once something goes viral on social media, it's difficult to stop its spread. And that has been raising questions about live broadcasting on social media and who should have access to it.

The alleged shooter seems to have first advertised the attack on the online forum 8chan, a message board known for right-wing extremist users.

He included a link to a Facebook account.

That Facebook account is where a 17-minute video was livestreamed in real time. The video starts behind the wheel of a car. It appears to come from a body-mounted camera. The alleged shooter pulls up to Al Noor Mosque, one of two mosques attacked in Christchurch, New Zealand. And what comes next is sheer horror. He starts shooting worshippers. At one point, he returns to his car for another gun. Then he shoots people who are quite close to him.

Professor Alex London teaches ethics and philosophy at Carnegie Mellon University. He says the viral nature of social media livestreaming makes it an ideal tool for extremist groups to spread their message.

"When your point is to strike fear into the hearts of people, livestreaming allows you to carry your message much farther," London said.

But that same technology and social media access also allow people to call out things like police brutality. For example, London said that it used to be that when people would accuse police officers of abuse, "you'd have to believe their testimony." Now, livestreaming provides more solid evidence in some cases.

Perhaps that's why, when Philando Castile was shot by a Minnesota police officer, his girlfriend's first instinct was to start broadcasting live on Facebook. The video went viral, causing national outrage. "It gives people a much better sense of the event, and the event in real time," London said.

But as much as livestreaming can document, it can also prop up atrocities and make them go viral, as in the case of the New Zealand shooting.

This is hardly the first time the role of social media in these types of attacks has been highlighted.

Last year, before an alleged armed shooter opened fire on a synagogue in Pennsylvania, he first posted hate speech on the fringe message board Gab, which has been accused of harboring extremist viewpoints. A Florida man accused of sending pipe bombs to various politicians across the United States had a history of threatening people on Twitter. And over the past year, Facebook has come under intense scrutiny for allowing hate groups on all its platforms, including WhatsApp and Instagram.

Recently, Facebook CEO Mark Zuckerberg announced that he was adding more moderators to crack down on disturbing content.

Katie Moussouris, a cybersecurity expert, is advocating that social media companies crack down on the ability by anyone in the public to livestream.

"It's not a bad idea to potentially have only verified accounts allowed to post. And if something that they post that is livestreamed does contain violence or hate speech, that privilege goes away," Moussouris said.

Professor Al Tompkins, who teaches ethics at the Poynter Institute, said free speech should be taken into account when considering the reaction to these tragedies.

"Look, here's the thing about free speech and free expression: It's a messy proposition, and there's always going to be abuse," he said. "It's true offline. It's true online."

Twitter and YouTube have both condemned the attacks and said they are working to bring down any video of the shooting.

Facebook said in a statement that it moved quickly to take down the shooter's Facebook and Instagram accounts, as well as the video. It also said it is removing any praise or support for the crime and shooter.

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