JM Berger, a researcher on extremism, points to a 2011 video clip that features Adam Gadahn, an American al-Qaeda recruit who was one of the most prominent voices in the group at the time.

A number of experts said that this appeared to be the first time the Islamic State or one of its former recruits had spoken of the organisation's views on US gun control. It isn't the first time that an Islamic militant group has suggested that the country's gun laws make it a ready target for attacks, however.

Suggestions that those on the terrorist watch list should be barred from purchasing weapons has led to a backlash from both the National Rifle Association and civil rights lawyers, who argue that the current terrorist watch list system is too flawed for a ban to work.

Those who favour greater gun control in the states have pointed to the availability of guns in the country as a risk factor, enabling easy and legal access to weapons that can kill a lot of people in a short space of time. Pro-gun voices, meanwhile, suggest that the widespread availability of guns leads to "good guys with guns" who could stop a terrorist shooter in his tracks.

"America is absolutely awash with easily obtainable firearms," Gadahn told potential al-Qaeda recruits. "You can go down to a gun show at the local convention centre, and come away with a fully automatic assault rifle without a background check and most likely without having to show an identification card. So what are you waiting for?" (Gadahn was killed last year in Pakistan by a CIA drone strike.)

At the time, Mr Berger wrote that this was "only the latest example of gun-love from al-Qaeda and its affiliates" and that "there are compelling reasons to focus our terrorism prevention strategy on firearms-based attacks." To an extent, this prediction has come true already: According to one recent analysis by FiveThirtyEight's Carl Bialik, an increasing number of terrorist attacks in the United States are committed with firearms, in part a response to stricter laws controlling explosives and materials that can be used to create explosives.

Speaking to the Times, Sarfo suggested that the Islamic State was actively seeking foreign fighters in Syria and Iraq to send back to Europe, where they could carry out attacks. Recruits with criminal histories were especially sought after by the group, "especially if they know you have ties to organised crime and they know you can get fake IDs, or they know you have contact men in Europe who can smuggle you into the European Union."

Weapons were also a factor. In much of Western Europe, gun laws are tight. However, the continent's largely open borders make smuggling easy, and guns can be obtained on the black market if you know who to ask. The Islamic State recruits who attacked Paris in November 2015 were found to have firearms that had originated in Eastern Europe. The Brussels-based cell that planned and perpetrated that attack and another in Belgium five months later had links to the criminal world, and some had spent time with the Islamic State in Syria and Iraq.

In contrast, an attack in San Bernardino, California, in December 2015 and another in Orlando in June 2016 - two attacks both claimed by the Islamic State that killed a total of 63 people - were conducted by people with little link to organised crime. These people had not been to the Middle East to fight and their links to the Islamic State were weak, at best. In both of these shootings, the firearms used were bought legally.