“He and other terrorists of various stripes see themselves as acting to protect their group when few others are willing to do so, partly in the hope that their actions can provide inspiration for others to do the same—just as Breivik inspired him,” Meleagrou-Hitchens wrote.

But terrorist propaganda comes in many forms on the internet. The Islamic State was famed for its “slick” videos glorifying violence; it used social media to recruit followers, and more actively to help incite and even coordinate attacks; and individual attackers like the Orlando, Florida, shooter posted pledges of allegiance to the group online.

Read: The Pittsburgh gunman embraced conspiracy theories. He’s not the first.

The global white-supremacist movement looks more atomized and diffuse than the jihadist movement, even as its adherents communicate with one another and sound similar themes. There is no “core territory” of white supremacy, as Afghanistan and Pakistan were for al-Qaeda in the 1990s and 2000s, or Iraq and Syria have been for ISIS today. This also means there is no military target to attack, the way French bombers pounded the de facto ISIS capital of Raqqa, Syria, following the Paris attacks.

Yet even here, there are similarities. In the United States, for example, most of the plots attributed to ISIS have had no physical connection to the “caliphate” in Iraq and Syria. Indictment data collected by the Program on Extremism show that the majority of those prosecuted for ISIS-related crimes in the U.S. were homegrown, American citizens or permanent residents who never traveled or even tried to travel to the Islamic “state” itself. No bombing of Syria was ever going to touch them directly. What worked to retaliate in those cases was law enforcement, the same way that the New Zealand police have apprehended suspects believed to be involved in the mosque attack.

The common challenge, however, is that there’s often no telling who is a threat until an attack has already taken place. In the United States since 9/11, an enormous amount of law-enforcement resources have been devoted to tracking and disrupting domestic jihadist plots. A recent New York Times investigation found that those resources far outstripped those devoted to tracking white-supremacist violence in the United States, which by some measures has been even more virulent than the jihadist threat when it comes to terrorist attacks on U.S. soil since 9/11. The news media’s focus, too, tends to be on attacks by Muslims—one recent study examining 136 terrorist attacks over a decade found that such attacks got three times more coverage than attacks of any other kind.

Adam Serwer: Homegrown terrorists in 2018 were almost all right-wing

Common to all these attackers, in the end, is the lives they destroy and the grieving loved ones their actions leave behind. Their political motivations, their online habits, their conspiracy theories, can’t ever fully explain a violence so fundamentally senseless, and a community loss so fundamentally cruel.