When an attacker plowed a vehicle into pedestrians on London’s Westminster Bridge on Wednesday, it was the latest in a string of incidents in which drivers used their vehicles as weapons.

Some were orchestrated by extremist groups, others by unstable individuals with unclear motives. While no one so far has claimed responsibility for Wednesday’s attack in London, radical groups have urged supporters to use any weapons at hand — including cars. The tactic presents a major challenge for security forces around the world.

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MARCH 22, 2017

A vehicle mowed down pedestrians on London’s Westminster Bridge, killing two people and leaving others with injuries described as catastrophic. Around the same time, a knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain’s Parliament. Authorities were treating the attacks as a terrorist incident.

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JAN. 20 , 2017

A man with a history of mental health and drug abuse issues drove into a street crowded with pedestrians in Melbourne, Australia’s second-largest city, killing at least four people, including a child, and injuring around 15 others. The 26-year-old man was arrested, and police said the incident had no links to terrorism.

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DEC. 2016

A young Tunisian rammed a truck into a crowded Berlin Christmas market, killing 12 and injuring dozens in an attack claimed by the Islamic State group. It was the first mass casualty attack by Islamic extremists carried out on German soil. Attacker Anis Amri, who had been denied asylum in Germany, was killed by police in Italy after an international manhunt.

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JULY 14, 2016

A Tunisian residing in France plowed a refrigerator truck through a crowd celebrating Bastille Day on the Mediterranean beachfront in Nice, killing 86. Attacker Mohamed Lahouaiej Bouhlel, who claimed allegiance to the Islamic State group, was killed by police. Cities around the world beefed up measures to prevent vehicle attacks in response.

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DECEMBER 2014

A motorist injured 13 pedestrians in the French city of Dijon; a day later, a man ran over pedestrians at a Christmas market in Nantes in western France, killing one and injuring nine. Both suspects, who survived, had histories of mental illness.

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OCT. 20, 2014

A 25-year-old man drove his car into to Canadian Air Force members near Montreal, killing one of them and injuring another. Authorities said the driver was a convert to Islam and had been flagged for jihadist ambitions. He was later shot dead by police.

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JUNE 30, 2007

Two men attempted to crash a blazing Jeep loaded with explosives into Glasgow Airport in Scotland. The car’s path was blocked and the explosives failed to detonate.

(AP)