(CNN)A vehicle rammed into pedestrians Friday on one of the busiest streets in Stockholm in what Swedish officials called a deliberate attack.
Vehicles as weapons: Stockholm attack is part of emerging terror tactic
The details remain unclear, but Stockholm joins a growing list of cities across Europe and around the world in which cars or vehicles have been used as weapons.
"Steal a lorry or a car and then drive it into a crowd," former Swedish Prime Minister Carl Bildt said in a tweet. "That seems to be the latest terrorist method. Berlin. London. Now Stockholm."
Here are some of the most significant recent attacks -- and the reasons behind them:
Date of attack: March 22, 2017
Number of casualties: Five people died in the attack, including an American man and an unarmed police officer, and scores of others were injured.
What happened: Police say an assailant rammed his rental car into pedestrians on Westminster Bridge, not far from the UK Parliament. The suspect then entered Parliament grounds and fatally stabbed a police officer before being shot dead by other officers.
Why it happened: The attacker, identified as 52-year-old British man Khalid Masood, acted alone and was inspired by international terrorism, officials said.
He had been convicted on a string of violent crimes and weapons charges, but officials said they weren't sure how he became radicalized.
"Clearly that's the main line of our investigation -- is what led him to be radicalized," said Mark Rowley, Britain's top counterterrorism officer. "Was it through influences in a community, influences from overseas or through online propaganda?"
Date of attack: July 14, 2016
Number of casualties: 84 people killed, more than 200 wounded
What happened: Authorities say Mohamed Lahouaiej Bouhlel used a 20-ton truck to strike hundreds of people in Nice, where large crowds gathered to watch Bastille Day fireworks.
After the truck barreled through the crowd for almost a mile, police shot and killed Bouhlel.
Why it happened: ISIS said the attack was retaliation for France's role in the fight against ISIS.
"The person who carried out the run-over in Nice, France, is one of the Islamic State soldiers and carried out the operation in response to calls to target nationals of the coalition which is fighting the Islamic State," the terror group said in a statement.
But French Interior Minister Bernard Cazeneuve said Bouhlel had no record of making militant statements and was not believed to be a member of ISIS.
"It seems he became radicalized very quickly," Cazeneuve said.
Date of attack: December 19, 2016
Number of casualties: 12 people killed, at least 48 wounded
What happened: A tractor-trailer rammed into a crowd at a bustling Christmas market, which was filled with holiday shoppers. The suspect, Anis Amri, was killed later in a shootout with police in Italy.
Why it happened: A video showed Amri pledging allegiance to ISIS, and the ISIS-affiliated Amaq news agency said the attack was carried out by "a soldier of the Islamic State" to target citizens of countries fighting ISIS.
But CNN terrorism analyst Paul Cruickshank said ISIS often uses that kind of terminology to refer to attacks by alleged sympathizers in the West.
"This should not be taken to mean the group is claiming it directed this attack," Cruickshank said.
Date of attack: November 28, 2016
Number of casualties: 11 people wounded
What happened: Abdul Razak Ali Artan, an Ohio State University student, rammed his car into a group of pedestrians on the campus. He got out and lunged at passers-by with a knife.
Moments later, an Ohio State University police officer fatally shot Artan after he refused to stop.
Why it happened: Authorities said they believe Artan was inspired by terrorist propaganda from ISIS and the late Yemeni-American cleric Anwar al-Awlaki, two law enforcement sources said.
In a Facebook post shortly before the rampage, the Somali immigrant said he was "sick and tired" of seeing fellow Muslims "killed and tortured," federal law enforcement officials said.
He urged America "to stop interfering with other countries, especially the Muslim Ummah," a term for Muslim people at large. "By Allah, we will not let you sleep unless you give peace to the Muslims," he wrote.
Date of attack: January 8, 2017
Number of casualties: Four soldiers killed, at least 10 people wounded
What happened: Authorities said 28-year-old Fadi Qunbar plowed into a group of Israeli soldiers on a popular promenade overlooking the walled Old City of Jerusalem.
Why it happened: The driver may have been an ISIS sympathizer, Israeli Prime Minister Benjamin Netanyahu said.
"All signs show he is a supporter of the Islamic State," Netanyahu said in January. "We know there is a sequence of terror attacks, and it's quite possible that there is a connection between them, from France, Berlin and now Jerusalem."
Date of attack: October 20, 2014
Number of casualties: One soldier killed, one soldier wounded
What happened: Police said Martin Rouleau Couture used his car to strike two Canadian soldiers walking in a strip mall parking lot in St.-Jean-sur-Richelieu, Quebec. After leading police on a chase, Couture got out of his car and was fatally shot.
Why it happened: Authorities said they believe Couture had been "radicalized." He was arrested in July 2013, the Royal Canadian Mounted Police said.
"When he was arrested, he was about to go to Turkey," police spokeswoman Martine Fontaine said. "We stopped him as he was about to leave Canada for terrorist actions. He was questioned when he was arrested."
But authorities lacked enough evidence to keep Couture in custody.
While not all vehicle attacks are linked to terrorism, groups such as ISIS and al Qaeda have called on followers to use trucks as weapons.
In fact, an al Qaeda magazine published an article in 2010 titled "The Ultimate Mowing Machine."
The article calls for using a pickup as a "mowing machine, not to mow grass but mow down the enemies of Allah."
It said a four-wheel-drive pickup is needed -- "the stronger the better."
"To achieve maximum carnage, you need to pick up as much speed as you can while still retaining good control of your vehicle in order to maximize your inertia and be able to strike as many people as possible in your first run," the article says.
John Miller, deputy commissioner of intelligence for New York police, has said ISIS calls on supporters to use cars as weapons if they have no other means of attack.
"The ISIS call, as well as that of other terrorist groups, has been to use what you have on hand," Miller said in 2015.
"And that means if you can make a bomb, you're a bomber. But if you can't, use a gun. And if you can't find a gun, use a knife. And if you can't find a knife, use a car. So when we look at that, that is a broad spectrum of threats, and it's something to prepare for."