That horror is clear not only to attendees, but to attackers. It’s not a surprise that since the Bataclan massacre, which introduced the concert as a tool of modern terror, there have been two bloody attacks on concerts, one in Britain and one in the United States. That there will be more is a virtual certainty. This is because terrorists and other perpetrators of mass violence, whatever their motives, see what works and adopt it. Widespread press attention, which is itself a main goal of mass-casualty attacks, brings public awareness to the methods and can help inform would-be future attackers.

(It is too early to know what motivated the shooter in Las Vegas, whom police identified as Stephen Craig Paddock and said killed himself. ISIS has claimed responsibility for the attack, though as of yet U.S. officials said they had uncovered no links between the shooter and the group.)

“In the United States, school shooters study other school shooters, in particular Columbine,” Peter Bergen, a terrorism scholar at New America, told The Washington Post after the Paris attack. “This is true of terrorists as well. They study tactics that have worked before.”

At that time, Brookings Institution scholar Will McCants warned that terrorism experts were concerned that Paris would augur a shift toward greater use of firearms instead of bombs. “That sounds counterintuitive, but when you’re building the bomb, there’s usually a lot of people involved and you have to buy material that the government has monitored, so it’s easier to identify and disrupt a plot that involves a bomb,” McCants told the Post.

Incidents like Las Vegas, the Pulse shooting, and the 2015 shootings in San Bernadino, California, confirm the lethality of guns, but the dissemination of new tactics has been most apparent over the last two years in the increase in attacks that use trucks or cars not as transportation for bombs but as weapons themselves.

As Colin P. Clarke and Louis Klarevas wrote in The Atlantic in June, the most recent iteration of the tactic seems to have originated in a 2010 issue of Inspire, the magazine published by Al-Qaeda’s Yemeni affiliate. In recent years, vehicle attacks have proliferated widely, from the Middle East to Europe to the United States. Some of the worst include Nice, France, Berlin, and London, though smaller attacks have killed and injured many people elsewhere, including cases that largely escaped widespread attention in places like Quebec or Columbus, Ohio.

The logic behind these attacks is straightforward enough: As authorities have hardened other systems, hijackings have become harder to execute. Guns can be difficult to acquire, depending on where one is. Bombs are dangerous and unpredictable. But almost anyone can get a vehicle and find a crowd. Many of these ramming attacks have been perpetrated by people who claim allegiance to ISIS, but who seem to have had little or no direct contact with ISIS commanders. It’s very easy to learn the technique and act on it without having to travel to a training camp or even undertake risky communication with wanted terrorists.