STOCKHOLM (AP) — Sweden's prime minister says a truck crash in Stockholm was an attack on Sweden. It's the latest in a string of incidents where drivers have used their vehicles as weapons. Some were orchestrated by extremist groups, others by unstable individuals with unclear motives. Radical groups have urged supporters to use any weapons at hand, including cars, a tactic that presents a major challenge for security forces around the world. Here's a look at some other attacks:

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March 22, 2017

A man identified by police as Khalid Masood ran his rented SUV into pedestrians on London's Westminster Bridge. Three people were killed on the bridge; a fourth person, a Romanian woman who was knocked into the Thames, died Thursday. Masood was eventually shot and killed by police, but not before stabbing a police officer to death on the grounds of Parliament. Police say Masood was inspired by extremist ideology, but that there's no evidence he had direct links to the Islamic State group or al-Qaida.

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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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Jan. 8, 2017

A Palestinian truck driver rammed his vehicle into a crowd of Israeli soldiers at a popular Jerusalem tourist spot, killing four people and wounding 17 in the deadliest single attack of more than a year of Israeli-Palestinian violence. It was the latest in a string of stabbings, shootings and vehicle ramming attacks in a wave of violence that erupted in the fall of 2015.

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Dec. 19, 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 later 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 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.