Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of " United States of Jihad: Investigating America's Homegrown Terrorists ."

(CNN) Terrorists have turned to a frightening weapon that often is within easy reach and is particularly hard to guard against: using cars and trucks as killing machines.

Once again the world saw images of what authorities are investigating as a terrorist attack conducted with a vehicle -- this time a truck in Berlin that plowed into a Christmas market Monday, killing at least 12 people.

It was a more lethal version of the terrorist attack three weeks ago by Abdul Razak Ali Artan, a legal resident of the United States, who rammed his car into a group of people, injuring 11 at Ohio State University. Artan then attacked the group with a knife before a police officer killed him.

Counterterrorism officials used to worry about truck bombs. Now they also have to worry about trucks and other vehicles being used as weapons, which can generate many casualties at crowded public events.

This was made vividly clear by the case of the terrorist who killed 86 people with a truck. The victims were celebrating the French national holiday on July 14 on the promenade in Nice on the south coast of France.

The use of this tactic was proposed by al Qaeda's Yemeni branch in its webzine, Inspire, in which it called on potential terrorists in the West to use trucks as a weapon. A 2010 article headlined "The Ultimate Mowing Machine" called for deploying a pickup truck as a "mowing machine, not to mow grass but mow down the enemies of Allah."

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Four years later, an ISIS spokesman encouraged similar vehicular attacks, saying that followers should use a car to "run over" the enemies of ISIS.

Seemingly inspired by this call, Canadian Martin Rouleau Couture rammed his vehicle into two soldiers in Quebec on October 20, 2014, killing one and injuring the other.

Palestinian terrorists targeting Israeli civilians have also frequently used vehicles as weapons.

What's particularly frightening is that Western civilians can just be going about their everyday business and get mowed down, whether they are at a holiday celebration in France, attending college in the United States, or on Monday visiting a Christmas market in Berlin.

Car and truck attacks are some of the hardest to guard against because they take no special training, they do not involve the preparation of weapons such as explosive devices, and they don't involve the purchase of military-style assault rifles, which in many Western countries are not easy to obtain. Yet as we have seen, these low-tech attacks can be quite lethal.