On Wednesday, March 22, Khalid Masood drove a rented Hyundai Tucson onto the sidewalk of the Westminster Bridge in London. He plowed into the unsuspecting pedestrians before coming to a stop, exiting the vehicle and wielding a knife. In just 82 seconds, Masood killed five people—four civilians and one police officer—and injured 50. He was eventually shot and killed by a nearby officer.

ISIS claimed responsibility for the attack, stating that Masood was a soldier acting in their name. London deputy assistant police commissioner Neil Basu, however, said he might have been a lone wolf attacker. Regardless, police suggest he was inspired and radicalized by ISIS propaganda found online.

In the wake of the attack, the police arrested a dozen individuals they believed were linked to Masood. As of Saturday, though, nine had been released due to a lack of substantial evidence. Masood left no note, no statement of motive or reason; the only potentially related communication police have found is a WhatsApp message he sent shortly beforehand.

This was not the first time Masood displayed violent behavior. Beginning in 2003, he served three years in prison for an assault on a local pub owner with a knife. While in prison, he converted to Islam and changed his name. His radicalization occurred swiftly thereafter. His second wife, Farzana Malik – or Isaq, as her surname has also been reported – fled from their home, insisting upon divorce when he tracked her down. She described Masood as a “psychopath,” and her family members described him as very violent and controlling.

In the last year or so, attacks using tactics similar to Masood’s have increased significantly. In December, Anis Armi killed a Polish truck driver and stole his truck, driving it into the Christmas Market in Berlin. He killed twelve people and injured 65. Last July, Mohamed Lahouaiej-Bouhlel drove a rented cargo truck into a crowd celebrating Bastille Day in Nice, France. Eighty-six people were killed and 434 were injured. Similar attacks involving vehicles have taken place in Quebec (October 2014); Valence, France (January 2016); and Jerusalem (this January).

Even the U.S. is not safe from these types of attacks. In November, an Ohio State student, Abdul Razak Ali Artan, rammed his car into pedestrians on campus and attacked other students with a machete. He injured eleven before he was shot and killed by a campus police officer.

ISIS has taken to promoting these sorts of attacks by its followers in its propaganda. The attacks are inexpensive, and very efficient in the sense that they often result in high casualties while gaining prominent media attention. In addition, they achieve the primary goal, terror, exceedingly well. ISIS encourages its followers to conduct them in busy areas, particularly tourist attractions, where they will likely attract a large international audience due to wider coverage. Attacks of this sort are also a means by which followers, or those inspired by ISIS or other terrorist organizations, can easily act in the name of their groups.

The biggest challenge for police in the wake of these incidents is preventing similar attacks in the future. Countering Violent Extremism (CVE) programs are still developing in many cities and countries. While London, for example, has a very successful program and is a hub for CVE research, it’s clear that some attackers still easily avoid detection until it is too late.

Professor Anthony Glees of the University of Buckingham suggests that rental car companies should inquire as to their customers’ motives and report any suspicious people to the police. Doing so will not prevent carjackings or attackers using their own cars. Nevertheless, the Federal Bureau of Investigation and the Department of Justice have encouraged similar reporting in the U.S., asking companies to tell the police about customers who are reluctant to provide personal information, pay in cash for large transactions, or appear overly concerned with the size and specifications of their vehicles.

While we may not have a tried-and-true method for preventing these types of attacks at the moment, progress is being made. As academics and think tanks conduct research to help in further development of both government-run and independent CVE programs, police and counterterrorism forces are working to prevent as many attacks as possible and stop the radicalization of people like Masood.