A white nationalist who murdered an estimated 49 Muslims in Christchurch, New Zealand, on Friday livestreamed a portion of his gruesome crime on Facebook, sending social media companies scrambling to contain the spread of the video.

Major social media platforms, including Facebook, YouTube, and Twitter, have terms of service prohibiting graphically violent videos. Officials worry that wide distribution of such videos boosts the profile of mass shooters and could inspire copycats. It can also be painful for victims' families.

"Our hearts are broken over today’s terrible tragedy in New Zealand," YouTube tweeted. "Please know we are working vigilantly to remove any violent footage."

But the Internet is a large-scale copying machine; limiting the video's spread has proven difficult. In the hours after the shooting, videos kept popping up on major social media sites—and the social media sites were slow to respond.

BuzzFeed's Ryan Mac, for example, reported that YouTube initially slapped a disclaimer on copies of the video stating that it "may be inappropriate for some users"—but then let users choose to watch it anyway. Twitter said it was banning the video, but Mac reports still being able to find it on widely followed Twitter accounts hours later. By Friday morning, Facebook, YouTube, and Twitter all seem to have scrubbed most copies of the video from their sites.

This kind of incident creates a particular challenge for Reddit, a social media site that has long been known for its strong free-speech policies. For years, Reddit flatly refused to censor content—including hate speech and graphic violence and nudity—that isn't allowed on other major social media platforms.

Over the last year, Reddit has softened its absolutist free-speech stance. But that has left it with a somewhat muddled and reactive approach to extreme content.

"Do not post content that encourages, glorifies, incites, or calls for violence," one Reddit policy states. Yet Reddit has long hosted a forum called WatchPeopleDie. It's exactly what it sounds like: people post videos showing people being killed in shootings, car crashes, bull attacks, and other gruesome incidents. Reddit's theory seems to be that merely depicting violence isn't the same thing as glorifying it.

Naturally, links to the New Zealand video began popping up on this forum, but the video apparently crossed the line into glorification of violence. Reddit began taking down links to the video and suspending users who persisted in posting links.

"Any content containing links to the video stream are being removed in accordance with our site-wide policy," Reddit said in a statement to The Washington Post.

Of course, it's never going to be possible to scrub this kind of content from the Internet altogether. Thousands of people have undoubtedly made copies of the video, and some of them will re-post it in the seedier corners of the Internet. It took me about 10 minutes to find a copy on Friday morning, more than 12 hours after the attack.