More organized militants still resort to guns or bombs when launching attacks in Europe, as attacks in Paris and Brussels showed. But the Islamic State and al-Qaeda have encouraged “lone-wolf attackers” and supporters to use weapons that are easier to obtain — vehicles, knives and even rocks. The Islamic State on Tuesday claimed responsibility for the Berlin truck attack via its al-Amaq propaganda arm.

“Since the [Islamic State] has taken heavy losses on the ground in Syria and Iraq and is losing territory, it has more and more changed its propaganda to trigger lone-actor attacks by practically anyone willing to do it,” said Daniel Koehler, the director of the German Institute on Radicalization and De-Radicalization Studies.

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Cars and trucks are a particular concern. The Islamic State’s propaganda magazine, Rumiyah, specifically recommended trucks as a way to inflict mass casualties in its November issue. “Though being an essential part of modern life, very few actually comprehend the deadly and destructive capability of the motor vehicle and its capacity of reaping large numbers of casualties if used in a premeditated manner,” the magazine said.

Abu Muhammad ­al-Adnani, the former chief spokesman for the Islamic State who was killed in an airstrike this year, also promoted the use of cars and other weapons of opportunity.

“If you are not able to find an IED or a bullet, then single out the disbelieving American, Frenchman or any of their allies. Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him,” Adnani said in a message in 2014.

Even before the attacks in Nice and Berlin, cars were on the rise as a terrorist weapon. Before attackers stabbed and killed British soldier Lee Rigby on a London road in 2013, they struck him with a car. In December 2014, France witnessed several vehicle ramming attacks at Christmas markets in which dozens were wounded.

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Security agencies have feared for years that the number of vehicle attacks would rise. A 2010 Department of Homeland Security report, for instance, warned of “vehicle ramming attacks — using modified or unmodified vehicles — against crowds, buildings” and other targets overseas. The report’s authors were particularly concerned that such attacks could occur without any prior training, as The Washington Post’s Joby Warrick and Souad Mekhennet reported in July.

The Nice attack in July created larger public awareness of the possibility of such assaults for the first time. “While the scale of the attack was unprecedented, the modus operandi of using cars for an attack was not entirely new,” researchers Petter Nesser, Anne Stenersen and Emilie Oftedal argued in a recent contribution to the Perspectives on Terrorism journal. They refer to al-Qaeda’s propaganda magazine, Inspire, which described vehicles as possible weapons six years ago.

“We have reached a stage where terrorist organizations want to create an environment in which they could hit anywhere at any time, using whatever method,” said a European security official, who spoke to Warrick and Mekhennet in July on the condition of anonymity to discuss sensitive assessments of terrorist strategy.

As the overall number of attacks in Europe has risen sharply over the past two decades, so has the percentage of plots being conducted by single individuals. Intelligence services in Europe are worried that attacks conducted by such individuals with vehicles, knives or machetes will be hard to prevent.

For intelligence services, those changes have further complicated their mission to stop more attacks before they can be carried out. With little or no communication between Islamic State operatives and supporters willing to conduct attacks in Europe, intelligence services are left in the dark about plans even if they are able to monitor chats on messaging apps like WhatsApp or Telegram.