Who was behind the jihadist attacks on Europe and North America?

They were also executed in part by former foreign fighters.

However, the two other most deadly attacks – those in Orlando in June 2016 and in Nice in July 2016 – were carried out independently by individuals without operational connection to a jihadist group.

These episodes demonstrate that terrorist sympathisers who never travelled to conflict areas and who act independently can be as dangerous as a team of highly trained militants.

Overall, links between attackers and jihadist groups operating overseas are not always easy to determine.

Of the attacks that have hit the West since June 2014, fewer than one in 10 was carried out under direct orders from the leadership of IS.

In some cases it can be difficult to tell, for example IS said it was behind both the Las Ramblas and Cambrils attacks, but it did not provide any evidence.

Nevertheless, the influence of IS can be clearly seen.

During or before the attack, six out of 10 perpetrators pledged allegiance to a jihadist group, almost always IS – which frequently claims responsibility.