It's a depressingly familiar tale. A vehicle slams into a group of pedestrians in a Western city and the terrorist driving the car then uses a knife to inflict further damage and is soon shot by police.
This time it was Wednesday's attack outside one of the most iconic buildings in the world, the Houses of Parliament in London.
Three victims and the attacker are dead and there are at least 40 injuries. It's the most lethal terrorist attack in the United Kingdom since al Qaeda directed four suicide attackers who killed 52 commuters on the London transportation system on July 7, 2005.
The Parliament attack is just one in a series of such relatively low-tech -- and hard to defend against -- terrorist attacks in the West over the past three years that have typically been inspired by ISIS, and occasionally also inspired by Anwar al-Awlaki, the American-born al Qaeda cleric.
On November 28, 2016, Abdul Razak Ali Artan, an 18-year-old legal resident of the United States whose family was originally from Somalia, used a car to mow down a group of people at the Ohio State University. Artan then attacked the crowd with a knife. He injured 11 people before he was killed by a police officer.
In a message that Artan had posted on Facebook just before the attack, he told readers to "listen ... to our hero Imam Anwar al-Awlaki." Awlaki is a cleric prominent in al Qaeda who was killed by a CIA drone strike in Yemen in 2011.
A month after the Ohio State attack, 12 people were killed when a large truck plowed into a crowd at a Berlin Christmas market. The attack was carried out by what ISIS termed "a soldier of the Islamic State." This formulation didn't mean that ISIS had any direct role in the Berlin attack, only that ISIS had inspired it.
Similarly, during Bastille Day celebrations in Nice, France on July 14, 2016, Mohamed Lahouaiej Bouhlel killed 84 using a large truck as a weapon. ISIS claimed that the Nice attack was carried out by one of its "soldiers," though French authorities said Bouhlel had no formal links to the group.
On October 20, 2014 Canadian Martin Rouleau Couture, an ISIS sympathizer, ran over two soldiers in Quebec with a vehicle, killing one and injuring another.
Using vehicles as weapons is a tactic that has often used by Palestinian terrorists to target Israelis, but in 2014 an ISIS spokesman had encouraged such vehicular attacks in the West, saying of ISIS' enemies, "Run him over with your car."
In 2013, two terrorists mowed down British soldier Lee Rigby with a car as he was walking down a street in London and then hacked him to death. In court, one of the terrorists described al Qaeda as "brothers in Islam."
Three years earlier, al Qaeda's Yemeni branch had encouraged its recruits in the West in its webzine, Inspire, to use vehicles as a weapon. An Inspire article headlined "The Ultimate Mowing Machine" called for using a vehicle as a "mowing machine, not to mow grass but mow down the enemies of Allah."
These attacks are hard to defend against in free societies where crowds will gather, as was the case for Bastille Day in Nice, or the Christmas market in Berlin, or students attending Ohio State -- and now the throngs of tourists and visitors that typically crowd the sidewalks around the Houses of Parliament.
Of course, Western countries cannot turn all of their heavily trafficked pedestrian areas into zones of walls and barriers, but law enforcement needs to have a deep understanding of who may be radicalizing before they carry out a lethal terrorist attack.
This is not an easy task, as some ISIS-inspired terrorists are radicalizing quite quickly before they take action.
According to an unpublished FBI study of 80 terrorist attacks and plots in the United States since 2009, those who typically have the most useful information about radicalization and potential acts of violence are peers and family members.
Enlisting the help of peers or family members is the best defense against the kind of attack we saw in London on Wednesday.