To some Britons, there was one obvious reason for this: Britain's strict gun laws. James Cracknell, a former rower and Olympic Gold medalist for Britain, wrote about how differently things might have turned out if Britain had a more gun-friendly culture.

Paul Danaher, a British journalist with the BBC based in Washington, was among many other Brits who expressed similar sentiments on social media.

Firearm ownership is heavily restricted in Britain. After a horrific mass shooting at a school killed 15 children and their teacher in 1996, the British government pursued legislative bans on assault rifles and handguns and dramatically tightened background checks for other types of firearms.

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“In terms of the types of gun that can be legally owned, background checks and the penalties for illegal possession and use, we are one of the strictest in Europe,” Helen Poole, a researcher with the University of Coventry who recently studied firearms across the European Union, told WorldViews in an email last year.

These measures have had a clear effect. As The Washington Post reported in 2013 a total of 200,000 guns and 700 tons of ammunition had been taken off the streets in the 17 years since the 1996 school attack. Legal gun ownership, which was always relatively low, is now dramatically lower than gun ownership in the United States.

Illicit gun ownership is also low in Britain, which does not have the porous land borders of many of its European neighbors. Matt Lewis, director of Arquebus Solutions, a company that helps governments track illegal firearms, told WorldViews that the country is also a “world leader” in managing illicit gun circulation.

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Evidence shows that with a limited supply, criminals are often forced to resort to desperate measures to acquire firearms. In 2014, the Birmingham Mail reported that gangs in the city had been using “plundered war trophies and collectible weapons, sometimes more than 100 years old.” During riots in the city in 2011, experts discovered that a late-19th-century St. Etienne revolver had been fired.

Such gun measures seem to be broadly popular in Britain. A 2010 poll from YouGov found just 4 percent of the country wanted gun control relaxed, a figure dwarfed by the 31 percent who thought all guns should be completely banned. Some figures on the right, such as former United Kingdom Independence Party leader Nigel Farage, have called for laws to be relaxed in recent years.

Gun crime is relatively low in the country, although it hasn't been eradicated. There have been some cases of domestic terrorism in which firearms were used — just last year, a British member of Parliament was shot and stabbed to death by a licensed firearm holder as she walked around her constituency. But the lack of firearms, and in particular weapons such as assault rifles, limits the options of would-be terrorists. The Westminster attacker's choice of weapons — a more readily available knife and a rental SUV — seemed to be born out of limited options.

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However, while the death count in London may have been relatively low, other vehicle-based attacks in France and Germany have shown how deadly the tactic can be. And other observers noted that the police officer killed in the attack, Constable Keith Palmer, was unarmed when he confronted a knife-wielding attacker — and that the attacker was ultimately stopped by another officer carrying a firearm.