The Times and others have pointed out that the attacks came three days after an ISIS spokesman, Abu Mohammed al-Adnani, appeared to call for heightened violence during the Muslim holy month of Ramadan in an audio message. Charlie Winter, a researcher at the London-based Quilliam Foundation, posted excerpts on Twitter:

O Muslims everywhere, we congratulate you on the blessed month of Ramadan arriving. We praise Allah for making us reach this significant month. Take advantage of this great opportunity, O slaves of Allah, hasten toward righteous deeds and search for the best of them. And the best of the deeds to draw closer to Allah is jihad. So hasten towards it, aspire to battle in this noble month. … No acts of worship are equal to jihad. And jihad in Ramadan is not matched by jihad in other months. So glad tidings to the one who spends Ramadan as a fighter for the cause of Allah. … So, O Muslims, embark and hasten towards jihad. And oh mujahideen [holy warriors] everywhere, rush and move to make Ramadan a month of disasters for the kuffar [unbelievers].

One way of thinking about the possible relationship between today’s attacks and Adnani’s call to action is through the work of jihadist military theorist Abu Musab al-Suri, an early associate of Osama bin Laden’s (Suri’s whereabouts are currently unknown, though some believe he is imprisoned in Syria). In a 1,600-page book posted to the web in 2005, Suri laid out a theory of “leaderless jihad” that encouraged individuals or small cells to stage attacks independently from one another. As The New Yorker’s Lawrence Wright wrote in 2006, Suri conceived of al-Qaeda less as an institution than as an idea: “‘Al-Qaeda is not an organization, it is not a group, nor do we want it to be,’ he wrote. ‘It is a call, a reference, a methodology.’” Suri, per Wright, “preferred to speak more broadly of jihad, which he saw as a social movement, encompassing ‘all those who bear weapons—individuals, groups, and organizations—and wage jihad on the enemies of Islam.’”

In a translation by the SITE Intelligence Group of Suri’s book, The Call to Global Islamic Resistance, the author described the relationship of “solo jihadi terror work” to the project of controlling territory “and establishing Allah’s law on it.”

The military theory of the Call to Resistance rises on the reliance of jihad on two aspects: Solo jihadi terror work and the covert work of small cells that are completely disconnected from each other. Participation in the open fronts of jihad whenever the circumstances allow it. And we direct attention to: Cellular or solo jihadi terror according to urban or suburban gang warfare is the basis in tiring the enemy and making him reach a state of collapse and retreat, Allah willing. As for the jihad of open fronts, it is the basis of controlling the land and freeing it and establishing Allah's law on it, with Allah’s assistance. Solo jihadi terror and gang warfare done by small cells paves the way for the other type (the jihad of open fronts), and helps it, and supports it. But without confrontation and control, we can establish no nation. And this is the strategic aim of the project of the resistance.

Wright traced Suri’s influence on Abu Musab al-Zarqawi, who led ISIS’s predecessor al-Qaeda in Iraq until Zarqawi’s death in 2006. One of Zarqawi’s associates was a suspected ringleader in the 2004 Madrid train bombings, which killed 191 people. “Zarqawi and his men were putting into action the vision that al-Suri had laid out for them,” Wright wrote, “small, spontaneous groups carrying out individual acts of terror in Europe, and an open struggle for territory in Iraq.”

More than a decade after the Madrid attacks, that model now seems applicable to the way ISIS, people claiming affiliation with ISIS, and even people only vaguely suspected of jihadist sympathies can terrorize people far outside the territory the Islamic State actually controls.