Friday’s slaughter in two New Zealand mosques played out as a dystopian reality show delivered by some of America’s biggest technology companies. YouTube, Facebook, Reddit and Twitter all had roles in publicizing the violence and, by extension, the hate-filled ideology behind it.

These companies — some of the richest, most technologically advanced in the world — failed to rapidly quell the spread of troubling content as it metastasized across platforms, bringing horrific images to internet users worldwide.

The alleged shooter, Brenton Harrison Tarrant, 28, also released a 74-page manifesto denouncing Muslims and immigrants that spread widely online. He left behind a trail on Twitter and Facebook that amounted to footnotes to his manifesto. Over the two days before the shooting he posted about 60 of the same links across different platforms, nearly half of which were to YouTube videos that were still active late Friday.

The horror began Friday morning in New Zealand, as Tarrant used Facebook to livestream his assault on Al Noor Mosque, one of two Christchurch mosques that he attacked and the scene of most of the 49 fatalities. Many hours later — long after the man and other suspects had been arrested — some internet users still were uploading and re-uploading the video to YouTube and other online services. A search of keywords related to the event, such as “New Zealand,” surfaced a long list of videos, many of which were lengthy and uncensored views of the massacre.

The almost instantaneous spread of online images from Friday’s shooting underscored how deeply entwined social media platforms have become, with savvy users moving content back and forth across platforms faster than the platforms themselves can react. It also was a reminder of the repeated inability of YouTube, the world’s biggest video site, to detect and remove some types of violent content, even though it has for years automatically flagged nudity and copyrighted music.

Tech companies “have a content-moderation problem that is fundamentally beyond the scale that they know how to deal with,” said Becca Lewis, a researcher at Stanford University and the think tank Data & Society. “The financial incentives are in play to keep content first and monetization first.”

The New Zealand massacre video, which appeared to have been recorded with a GoPro helmet camera, spread particularly widely on YouTube as the people uploading it outraced website moderators’ ability to delete the clips. Some of the videos were named after quotes from the shooter, such as, “Let’s get this party started.”

On message boards such as Reddit, people posted links to the videos, which would then sometimes get deleted, only for others to post new links to alternative “mirror” sites, beginning the cycle anew.

Reddit, one of America’s most popular websites, on Friday banned forums named “gore” and “watchpeopledie,” where the videos had been reposted for users to narrate and comment on in real time. A moderator on the “watchpeopledie” forum had defended keeping the video online because it offered “unfiltered reality.” The seven-year-old forum had more than 300,000 subscribers as of the time of the New Zealand shooting.