Until 1904, most ships that encountered trouble at sea effectively vanished. "Ships at sea out of visual range were very much isolated from shore and other ships," writes the Telegraph Office, an online history of Morse code. "A ship could vanish from the high seas, and no one would know until that vessel failed to make a port connection." But by 1904, many ships were equipped with some of the earliest wireless radios. For the first time, they could signal their distress, bringing help from the shore. A handful of distress codes was eventually whittled to one: three dots, three dashes, three dots. SOS.

This week the world has been horrified — and captivated — by videos showing the deaths at the hands of police of Alton Sterling in Louisiana and Philando Castile in Minnesota. Last night, at a demonstration protesting those deaths and the broader trend of police violence toward black people, a sniper killed five police officers and wounded seven more. Sterling’s death was captured in videos shot on mobile phones and shared later. But the deaths that came later came to us live, via Facebook’s seven-month-old live-streaming tool. Facebook came late to live-streaming, after it was popularized last year by Meerkat and Twitter-owned Periscope. But Facebook’s ubiquity has made it the go-to app for anyone who suddenly becomes a witness to violence or its aftermath. In just a few weeks, it has become a new kind of SOS.

Consider Diamond Reynolds, Castile’s girlfriend, who responded with nearly unimaginable composure after he was shot after being pulled over for having a broken tail light. She began broadcasting on Facebook Live to describe the circumstances of Castile’s shooting — because, she said, she did not trust the justice system to investigate it. She used the only tool at her disposal — she turned her camera on not to share her pain, but to cry out for help. "I wanted everyone in the world to know that no matter how much the police tamper with evidence, how much they stick together … I wanted to put it on Facebook and go viral so that the people could see," Reynolds told reporters Thursday.

"I wanted to put it on Facebook and go viral so that the people could see."

In that sense, her stream had the desired effect. It has been viewed more than 5 million times, and helped supercharge the national conversation about police brutality. It barged into the sterile desert of the News Feed and confronted us with the persistent danger of being black in America. It bore bloody witness to a senseless killing and its bloody aftermath. By bringing us inside her car, by placing us next to her dying lover, Reynolds showed us a world that white people rarely have to visit. Implicit in the stream was a question: what are you going to do about it?

These are questions that seem not to have been anticipated by anyone who built the tools. Meerkat, which sparked new interest in live-streaming after many earlier attempts had failed, touted the app’s ability to promote "spontaneous togetherness." Its selling point wasn’t the live stream itself, but the interactions it enabled: broadcasters reacting to their audience in real time, enabling a new kind of live performance. Periscope, which arrived a few weeks later, used its tight integration with Twitter to take this even further. The media’s heavy usage of Twitter ensured that Periscope would soon be used to broadcast news, and it was — turning every reporter into a broadcast journalist, when called for, at the tap of a button. That was game-changing for reporters, but not for average people. "Citizen journalism" is much discussed but has never made a sustained impact in this country. And even when social media did upend regimes, during the Arab Spring, we looked upon it with pride as a validation of Western values — with little thought for what those tools would expose at home.

Facebook charged headlong into live-streaming a few months later, driven by its years-old wish to siphon away the billions of dollars in advertising money that are dedicated to television. If it envisioned Facebook Live as the future default platform for citizen journalism, the company didn’t mention it. "Live lets you show the people you care about what you’re seeing in real time," the company said in a cheery blog post introducing the feature, "whether you’re visiting a new place, cooking your favorite recipe, or just want to share some thoughts."

"I hope we never have to see another video like Diamond's."

But of course Facebook Live was destined to become a home for citizen journalism. It is part of an app used daily by 1.09 billion people. It offers an instant connection to family, friends, and a global audience. And it presses us relentlessly to share whatever we are doing and seeing. That we share has always been more important to Facebook than what we share. But in handing a live-streaming tool to a billion people, Facebook invited us to share darker, more disturbing stories than it has ever before hosted on its servers.

Suddenly, executives vision of Facebook as a whimsical place for cooking your favorite recipe is challenged by one that makes it look a lot more like a news channel — just as the company tries to tilt itself back to promoting status updates and photos from your friends. When Facebook executives say they expect the entire News Feed to become video in a few years, it’s hard to imagine this is what they wanted. What if all these distress calls drive people away from the News Feed? (I confess to having actively avoided watching many of these snuff films; it’s often all I can do to read the descriptions.)

For his part, Facebook CEO Mark Zuckerberg offered a tepid embrace of live-streaming used in this way, adding: "I hope we never have to see another video like Diamond’s." It’s a curious sentiment given that we almost certainly will; US police officers shot and killed 965 people last year alone. Facebook now faces important questions about how it will respond to its new status as the default destination for eyewitness accounts. It did not respond to a request for comment from The Verge today, but later posted a statement about its standards for live video. "Just as it gives us a window into the best moments in people's lives, it can also let us bear witness to the worst," the company wrote. "Live video can be a powerful tool in a crisis — to document events or ask for help."

Last month, House Democrats used Periscope (and Facebook Live, to a lesser extent) to broadcast their filibuster in favor of gun control. They turned to Periscope because the Republican majority, which controls access to the House’s cameras, turned them off. The smartphone was a leveling force, eliminating the Republicans’ comparative advantage. In a dark year, it felt like a bright moment: technology drawing attention to a vital subject, despite the best efforts of the ruling party to quash the discussion.

Of course, members of Congress are still elites, and have many tools at their disposal to draw attention to their pet causes. Diamond Reynolds’ broadcast this week, and the broadcasts that came after, represent a far more important redistribution of power. A woman picked up her phone and asked for our help live, in the moment, when she needed it the most. Help did not arrive in time to save Philando Castile, but Reynolds’ live-streamed distress call was heard around the world. The frequency of those distress calls is only going to increase. Facebook now has to decide what, if anything, to do about it. And so do we.

Update July 8th, 6:20PM: This article has been updated to include Facebook's statement.