As the Christchurch gunman live-streamed the massacre onto Facebook, fewer than 200 people watched.
The social network said its moderators removed it sometime after a user first reported it as troubling, 29 minutes after the stream began.
But on the anonymous message board 8chan, where the gunman had announced his attack with a link to the live footage, a nameless group had already been racing to save, preserve and reupload the video in corners of the web where it'd be harder to take down.
It worked: Within 24 hours, the video of a mass terror attack that killed 50 people at two mosques had been uploaded onto Facebook more than 1.5 million times.
New numbers released by social media companies confirm what has been apparent since the Friday killings: A single video that had been seen live by a relatively small audience had become, at the speed of the web, impossible to contain, propelled by the same social-media infrastructure that has helped make American tech one of the most popular and wealthiest industries in history.
The companies have pledged to devote more resources to content moderation. But the new details reveal just how quickly the video rocketed out of their control to become one of the most distressing and hauntingly intimate depictions of mass killing ever recorded.
The original 17-minute live stream was announced on 8chan, a site known as a haven for far-right extremism and hate speech, and posters there shared tips on how to save and share the video for "posterity" and maximum distribution.
The ease with which anyone could save and reupload it online, using a vast network of underground "mirror" sites, ensured that it would be permanently stamped across the web.
Facebook officials said they took down the original stream after it had been watched about 4000 times.
They also added the video to an internal ban list and began blocking it almost immediately, removing 1.5 million videos of the shooting within the first 24 hours, the company said.
About 1.2 million of those videos were blocked at the time of upload by content moderators or automated algorithms that can find and flag blacklisted videos.
But Facebook, which has more than two billion active users, has not said how many people viewed, commented on, reacted to or shared those 300,000 copies of the video once they were posted to the site.
The video took off on other platforms, including Twitter and YouTube, where unedited footage of the fatal shootings was viewable hours after the attack. YouTube officials told the Washington Post that a new copy was being uploaded every second.
To evade the tech giants' automatic blocking systems, people also began tweaking the video slightly and reuploading it. According to the Global Internet Forum to Counter Terrorism - a trade group formed by Facebook, Microsoft, Twitter and YouTube - more than 800 "visually distinct videos" of the attack had been digitally fingerprinted and banned.
The spread of the violent video has drawn sharp rebukes in Washington, where the House Homeland Security Committee, led by Representative Bennie Thompson, D-Miss., asked Facebook, Microsoft, YouTube and Twitter for a briefing on the matter next week.
In a note to those companies' top executives, sent today, Thompson stressed, "You must do better." He also threatened possible regulation on the horizon. "If you are unwilling to do so, Congress must consider policies to ensure that terrorist content is not distributed on your platforms - including by studying the examples being set by other countries," he said.
Prime Minister Jacinda Ardern said that the social-media giants must account for their role in aiding the videos' rapid spread.
"We cannot simply sit back and accept that these platforms just exist and what is said is not the responsibility of the place where they are published," Ardern said.
"They are the publisher, not just the postman. There cannot be a case of all profit, no responsibility."