When Koeberle Bull received a hateful Facebook message last week from a man who lived more than 500 miles away, she called the police. Dylan Jarrell in Kentucky, who had sent the New Jersey mom of three a violent and racist Facebook message directed at her children, was pulling out of his driveway with hundreds of rounds of ammunition and a plan to commit a school shooting when the local cops stopped him.

Because Bull took Jarrell’s violent racism and online harassment seriously — unwilling to brush it off as a remote threat — she likely saved lives. It’s a good lesson for us all and a reminder of that violent expressions of racism and sexism online can be harbingers of violence committed in the flesh. Unfortunately, how things went with Jarrell is the exception rather than the rule.

Instead, acts of online hate get brushed off as a heated political discourse, leaving victims of harassment (usually women and people from otherwise marginalized groups) to be canaries in the coal mine — people whose warning cries go ignored.

It’s now common to perform a kind of online psychological forensics search after horrible crimes are committed. That’s how we learned that, a few weeks before Cesar Sayoc mailed more than a dozen bombs to politicians and media organizations, the 56-year-old Florida man sent a threatening tweet to political analyst Rochelle Ritchie: “We will see you 4 sure. Hug your loved ones real close very time you leave you home.”

When Ritchie reported the threat, Twitter responded that Sayoc hadn’t violated their rules about abusive behavior. “You think to yourself, ‘If you see something, say something,’ and then when you say something, it’s ignored,” Ritchie told CNN. (Twitter later issued a statement acknowledging the company had made a mistake.) It’s also how we learned that the man who killed 11 people in a Pittsburgh synagogue in late October also frequently posted anti-Semitic threats online.

Another mass murderer, Elliot Rodger, posted multiple videos on YouTube raging against women who had sexually rejected him, threatening to “slaughter… every blond slut I see,” before he went on his rampage.

And yet online harassment and hateful speech still isn’t taken seriously until something even worse happens.

But the truth is there’s no such thing as “just a tweet” anymore. Especially not now, when the president of the United States uses social media to dog-whistle his followers’ ire. And anyway, there is ample evidence that violence and harassment — be it online or off — often go hand in hand.

Sayoc, the attempted mail bomber, and Gregory Bush, a man who killed two black people last week, also had a history of domestic violence. (The same is true for nearly every mass shooter in recent U.S. history.) Before a high school shooter in Texas killed 10 of his classmates, he spent months harassing a girl in one of his classes. The man who killed five people this year at Maryland’s Capital Gazette stalked a woman he knew as a teenager.

Imagine if instead of excusing violence and everyday sexism, we took them seriously. Imagine if a boy in a classroom who repeatedly touched a girl without permission was something that sparked a serious intervention rather than a “boys will be boys” shrug. Imagine if we listened to the people of color who have been raising the alarm about Trump’s racist speech (only to be proven right in the worst way possible). Imagine if we acknowledged the danger inherent in white supremacist speech.

Because a country that takes racism and sexism seriously is a country that saves lives.

Red flags are everywhere — and we can’t afford to keep ignoring them as if lives don’t depend on it. They do.