KELLY MCEVERS, HOST:
We're starting to get some details about Khalid Masood. He had a criminal record. He had been arrested for assault, possession of, quote, "offensive weapons" and for public order offenses. But police in the U.K. say they had no prior intelligence about his intent to mount a terror attack.
AUDIE CORNISH, HOST:
ISIS is now claiming responsibility. The group has been encouraging low-tech attacks like this using vehicles and knives and has offered its supporters detailed guidance on the internet. NPR's national security correspondent Greg Myre has more on that.
GREG MYRE, BYLINE: The Islamic State's online magazine spells out the do's and don'ts of a vehicle attack in a recent issue. The group recommends a load-bearing truck, one with double wheels if possible and tells supporters to avoid small cars that don't accelerate well. The bullet points go on and on and then turn to a detailed list about what kind of knife to use.
Daniel Byman, a counterterrorism analyst at Georgetown University, says these recent attacks in Western countries are becoming more important to ISIS as it loses territory on the battlefields in Iraq and Syria.
DANIEL BYMAN: In general, these less-skilled attacks tend to be less lethal. But at the same time, it sows fear. It also creates the impression that the group is active, that it is striking back.
MYRE: Europe has suffered five major terror attacks in the past 16 months, and the Islamic State has claimed all of them. The three most recent ones have all involved vehicles - yesterday in London, a Christmastime attack in Berlin and a rampage last July at a fireworks display in Nice, France.
Extremists are having a much harder time going into the core ISIS areas in the Middle East where thousands of recruits flocked in recent years. Byman says the group wants to turn this limitation into an advantage by telling supporters to stay at home and strike there.
BYMAN: In the short term, terrorism is relatively cheap, and they could use their limited resources to do terrorist attacks rather than defending their territory as they're doing now.
MYRE: The ISIS magazine was known as Dabiq, which is the town in Syria where the group believes an apocalyptic end-of-days battle will be waged. But ISIS fighters were chased out of Dabiq last fall with no such drama. Still, that defeat did prompt the group to change the magazine name to Rumiyah, which refers to Rome, as in Rome, the Italian city ISIS dreams of capturing.
Doug Ollivant, an analyst with New America, said these vehicle and knife attacks are more likely in Europe where it's much harder to get a gun.
DOUG OLLIVANT: We probably will see fewer attacks in America with cars and knives because firearms are so ubiquitous here. Why use a truck when it's pretty easy to get your hands on firearms?
MYRE: However, these kind of attacks have surfaced in the U.S. as well. One was a car and knife assault at Ohio State University last November. There was also a stabbing rampage in a Minnesota mall in September. In both cases, there were multiple injuries, but only the attackers were killed. These low-tech, one-man operations are hard to detect and are in sharp contrast to the carefully planned mass attacks that ISIS and al-Qaida carried out in the past.
But more sophisticated threats have not been eliminated. Britain and the U.S. announced Tuesday that passengers on flights from the Middle East and North Africa would not be allowed to carry on electronic devices larger than a smartphone. The fear is that a terrorist group may have a bomb that could be disguised as a laptop battery. Greg Myre, NPR News, Washington. Transcript provided by NPR, Copyright NPR.