Years before he killed two women at a yoga studio in Tallahassee, Florida, Scott Beierle was posting YouTube videos in which he called women “sluts” and “whores” and talked about ripping their heads off.

Beierle, who killed himself on Friday after killing Nancy Van Vessem and Maura Binkley and wounding five others at Hot Yoga Tallahassee, had filmed several virulently misogynist videos in 2014, according to David Mack, Amber Jamieson, and Julia Reinstein of BuzzFeed News. He had also uploaded sexist and violent songs to Soundcloud in the last few months.

In the videos, Beierle rants about women who canceled dates or gave him their phone numbers even though they had boyfriends. He also mentions Elliot Rodger, the gunman who killed six people and wounded 13 in Isla Vista, California in 2014 — and who has become a hero to men who identify as “incels” (short for “involuntary celibate”).

The Tallahassee shooting was the third crime in a single week that was apparently preceded by a trail of online hate. Robert Bowers, the man suspected of killing 11 people and wounding six others in a shooting at a Pittsburgh synagogue last Saturday, appears to have posted threatening language about Jewish people and HIAS National Refugee Shabbat, a refugee aid group formerly known as the Hebrew Immigration Aid Society, on Gab, a social network that has become a home for anti-Semitism and racism.

Cesar Sayoc, the man arrested last Friday in connection with bombs mailed to CNN and several critics of President Donald Trump, appears to have threatened Democrats on Twitter and Facebook. “Hug your loved son,Niece,wife family real close everytime U walk out your home,” said one tweet sent to former Vice President Joe Biden, apparently by Sayoc. It included an image of Biden’s home with a target superimposed on it.

The postings apparently made by Beierle, Bowers, and Sayoc were part of a pattern going back years. Elliot Rodger left behind a YouTube video in which he said women would be punished for not being attracted to him. And George Sodini, a gunman whose murder of three women at a gym outside Pittsburgh in 2009 appears to share similarities with the Tallahassee shooting, maintained a blog detailing his anger toward women for months before committing the crime.

These men share more than their apparent online histories of bigotry. All were part of communities, online or off, that seemed to reinforce their views. That’s why online hatred and harassment is so serious. It’s not just that an individual person’s online posts can be warning signs of future violence. It’s also that hateful posts, even by those who never commit crimes, create an environment where those crimes are encouraged, accepted, and even celebrated.

Many mass murderers and other criminals have left behind an online trail

Beierle’s YouTube and SoundCloud history is rife with violent sexism. In one video, he says of a woman who canceled dates with him, “I could have ripped her head off,” according to BuzzFeed. In a song called “Locked in My Basement,” he describes holding a woman prisoner and raping her.

He mentions Rodger in a video called “Plight of the Adolescent Male,” saying, “I’d like to send a message now to the adolescent males ... that are in the position, the situation, the disposition of Elliot Rodger, of not getting any, no love, no nothing. This endless wasteland that breeds this longing and this frustration. That was me, certainly, as an adolescent.”

Beierle had also been arrested in 2012 and 2016 for grabbing women’s buttocks without their consent, according to the Tallahassee Democrat.

Meanwhile, posts apparently by Robert Bowers, the alleged Pittsburgh assailant, reveal a history of anti-Semitism, and a web of influences. On Gab, someone by the name of Robert Bowers posted a variety of anti-Semitic slurs and statements, including “jews are the children of satan,” according to the Associated Press. The same user criticized President Trump for being insufficiently anti-Jewish, and bragged about his gun collection, calling it his “glock family” and saying one gun in particular had an “amazing trigger.”

As Vox’s Jane Coaston notes, Gab was started as a “free speech” alternative to Twitter, and allows forms of hate speech that are banned on more mainstream social networks. Because of this, it’s become a gathering place — and recruitment forum — for neo-Nazi groups. One post from a group called Atomwaffen Division features swastikas, racial slurs, and the invitation, “Join your local Nazis.”

The synagogue shooting suspect also acted out of a specific kind of anti-Semitism, as Vox’s Dara Lind points out: “blaming Jews in America for bringing in an invasion of nonwhite immigrants who would slaughter the white race.” This particular kind of bigotry has been fueled by Trump’s claims that his political opponents are at fault for the caravan of migrants approaching the US, and by anti-Semitic conspiracy theorists on Gab and other social networks, who have cast the caravan as a Jewish “invasion.”

Most of the people posting this theory online have never committed anti-Semitic violence, but they helped create the conditions under which something like the Pittsburgh shooting could occur.

Twitter and Facebook accounts connected to Sayoc contain threats against Biden and former Attorney General Eric Holder, both of whom received pipe bombs. CNN analyst Phil Mudd also received a Twitter threat apparently from Sayoc. Political analyst Rochelle Ritchie reported a threat apparently from Sayoc to Twitter in the days before the shooting, but Twitter did not take action.

Sayoc was apparently a devotee of a variety of conspiracy theories, many of which have spread on Twitter. As Vox’s German Lopez points out, he often posted criticisms of liberal billionaire George Soros, a common target of anti-Semitic fear-mongering on Twitter and elsewhere. Trump has fanned the flames of this fear-mongering, retweeting a claim earlier this year that Soros was a “nazi who turned in his fellow Jews.”

Beierle, Bowers, and Sayoc were far from alone in leaving what appear to be online trails pointing to future violent behavior.

“Tomorrow is the day of retribution,” Rodger said in his last YouTube video, according to the New York Times. “For the last eight years of my life, ever since I hit puberty, I’ve been forced to endure an existence of loneliness, rejection and unfulfilled desires. Girls gave their affection and sex and love to other men but never to me.”

“I do not know why you girls aren’t attracted to me,” he added. “But I will punish you all for it.”

Rodger also identified as an “incel” and posted on forums populated by men with similar frustrations. The 22-year-old, who took his own life after killing others, became something of a hero to other “incels,” as Vox’s Zack Beauchamp has noted. A man who killed 10 people by driving a van down a Toronto street in April identified himself on Facebook as part of the “Incel Rebellion.” Also on Facebook, he praised the Rodger, calling him the “Supreme Gentleman,” a nickname Rodger had chosen for himself.

The man who shot and killed three women at a Pittsburgh gym, meanwhile, wrote on his personal website in 2008, “I dress good, am clean-shaven, bathe, touch of cologne – yet 30 million women rejected me – over an 18 or 25-year period.”

“A man needs a woman for confidence,” he added. “He gets a boost on the job, career, with other men, and everywhere else when he knows inside he has someone to spend the night with and who is also a friend. This type of life I see is a closed world with me specifically and totally excluded.”

He also had ties to the pickup artist community, having purchased the book How to Date Young Women: For Men Over 35, and appearing in a video of one of author R. Don Steele’s lectures. Pickup artistry, more in vogue in the early 2000s than it is now, sometimes treated women as less human than men, as “targets” to be coerced or fooled into sex. It’s not murder, of course, but this was the climate of misogyny in which he planned his crime.

Fighting hate crimes means understanding the communities that support them

Each of these men may have seemed like a loner — but in fact, each was part of a group of people with similarly toxic views directed at Jewish people, women, or people of color. And we need to understand crimes like the Tallahassee and Pittsburgh shootings and the mailing of pipe bombs to Democrats within their larger context of on- and offline hate.

That means holding platforms accountable for the behavior they allow. In the wake of the pipe bombings, Twitter has apologized for not taking Ritchie’s report more seriously. “We are investigating what happened and will continue to work to improve how we handle concerns raised by anyone on Twitter,” the account @TwitterSafety tweeted last week.

But in response to a request for specifics from The Verge, the company merely pointed to blog posts from earlier in the year. “This is a familiar story from Twitter,” the Verge’s Andrew Liptak writes: “apologizing for reacting after the fact after it becomes clear that someone violated the site’s terms.”

Gab, meanwhile, is being more defiant. “Gab.com is under attack,” read a message posted by the company on Monday. “We have been smeared by the mainstream media for defending free expression and individual liberty for all people and for working with law enforcement to ensure that justice is served for the horrible atrocity committed in Pittsburgh.”

Investigating and prosecuting online threats can be tricky because perpetrators can conceal their identities and locations, and because when someone in one part of the country threatens someone in another, it’s not always clear which law enforcement agency should take the case.

But there’s evidence that law enforcement, even at the federal level, isn’t doing all it can: The Department of Justice prosecutes only a small minority of cases of online threats and stalking, Joshua Eaton reported at ThinkProgress last year. A bill that would provide resources to help the FBI, DOJ, and local authorities fight such crimes, introduced by Rep. Katherine Clark (D-MA), has been stuck in committee since last July.

While law enforcement can prosecute threats and stalking, other hate speech isn’t illegal. But that doesn’t mean companies like Twitter have to allow it on their platforms. As journalist Stacy-Marie Ishmael pointed out on Twitter, women, and especially black women, who experience disproportionate harassment online, have been warning of the seriousness of this harassment for years. Their warnings have mostly fallen on deaf ears.

Taking online hate seriously would require platforms like Twitter to make fighting threats and bigotry a core part of their mission, not an afterthought. As Kate Klonick wrote at Vox in 2016, “Twitter needs to view fighting abuse as an essential feature.”

It also means law enforcement, government, and ordinary users need to be aware of the ways in which online communities can fuel offline hate. Men like Beierle, Bowers, and Sayoc have been posting about their violent intentions for years now, and getting support and affirmation for doing so. It’s long past time to start paying attention.