ISIS recently called for the type of attack that just happened in Berlin

German investigators are treating the incident as an act of terrorism. The White House on Monday called it a terrorist attack, and Merkel said German authorities were investigating under the assumption that terrorism was involved.

There has been no claim of responsibility for the Monday-night carnage. The German police have one suspect in custody, but it is unclear whether he was the truck driver.

Terrorist groups have long been urging supporters to attack "infidels" with cars, however. Vehicles were used in attacks at Ohio State University in the US late last month and in Nice, France, in July.

In November, an Ohio State student drove into a group of pedestrians on campus before stabbing people with a butcher knife. Eleven people were sent to the hospital with injuries. A campus police officer killed the attacker at the scene.

ISIS claimed responsibility for the attack, and supporters of the group were spreading information about it through online channels shortly after it happened. The attacker, 18-year-old Abdul Razak Ali Artan, a US resident from Somalia, was thought by authorities to have cited the late Al Qaeda cleric Anwar al-Awlaki on Facebook.

While it's still unclear whether any one group inspired the Ohio State attack, terrorism analysts were quick to note that it closely mirrored recent ISIS propaganda.

In the past two months, English-language ISIS propaganda magazines have called for vehicle and knife attacks on Western targets. The November issue of Rumiyah, for example — a new magazine from the terrorist group aimed at English-language speakers — included an article titled "Just Terror Tactics" that outlined ideal vehicles to use in terror attacks.

"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 article said.

The article also cited the attack in Nice in July, in which a man who authorities said was an ISIS supporter killed 86 people by plowing into a crowd with a truck on Bastille Day.

"Vehicles are like knives, as they are extremely easy to acquire," the article said.

Abu Mohammad al-Adnani, the ISIS spokesman who was killed in August, had also called for these types of attacks.

"If you are not able to find an IED or a bullet, then single out the disbelieving American, Frenchman, or any of their allies," he said in 2014. "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, or poison him."

Al Qaeda has also put out global calls to attack Westerners with cars.

In the second issue of its English-language magazine Inspire, the terrorist group referred to pickup trucks as "the ultimate mowing machine."

"The idea is to use a pickup truck as a mowing machine, not to mow grass but mow down the enemies of Allah," the magazine article says.

ISIS, in particular, has increasingly been relying on external attacks as it has been losing territory in the Middle East, where its self-declared caliphate lies.

When the terrorist group first rampaged across Iraq and Syria claiming territory, it encouraged supporters to travel to territory under its control, but recent ISIS rhetoric has shifted to focus on encouraging people to mount attacks in their home countries.

Sometimes these attacks are directed by ISIS leadership, but sometimes they are carried out by lone actors who don't have any significant contact with ISIS members.

ISIS is likely to claim a role in the Berlin attack only if it is publicly confirmed that the truck driver had an ISIS allegiance, said Rita Katz, the director of the SITE Intelligence Group, which monitors terrorist organizations.

The German police took a suspect into custody Monday night in connection with the Berlin attack, and reports circulated that he was a 23-year-old from Pakistan who had sought asylum in Germany in early 2016. He was known to the German police because of petty crimes but did not have any known links to terrorism.

The police subsequently said they were unsure whether they believed the suspect in custody was the driver of the truck.

"We have the wrong man," a high-ranking police official told Welt.de. "And thus a new situation. As the real culprit is still armed at large and can cause new damage."