But the tactics of the assailants seem to fit “urban warfare” style of attack employed in the Charlie Hebdo shooting in January, and the weapons reportedly used by the assailants, AK-47s, match those used in that incident as well as numerous other shooting incidents in France in recent years.

Even before Charlie Hebdo, France was on edge over a number of attacks linked to Islamic extremists, and hundreds of French citizens have already traveled to Syria to fight with groups including ISIS. Since January, there have also been a number of smaller incidents, including a decapitation and attack on a gas factory by an alleged ISIS sympathizer near Lyon in June, the arrest of a student with a cache of Kalashnikovs allegedly planning an attack on Paris churches in April, and a foiled attack on a high-speed train in August.

Attacks like these create something of a feedback loop. Fears of terrorism contribute to the suspicion and marginalization of France’s large Muslim population (you can almost surely expect support for Marine Le Pen’s National Front to grow after an incident like this), which then contributes to radicalization. The Charlie Hebdo attack also prompted France to take a more active part in the war in Syria, launching airstrikes against ISIS in September, another potential pretext for the shooters, according to some unconfirmed early reports. The attack also comes after a number of battlefield setbacks for ISIS, ahead of key talks on Syria this weekend by a number of countries including France, and just after a devastating bombing claimed by ISIS in Beirut on Thursday.

If there is a link to Syria or events in the Middle East—which again, we do not yet know for sure—Friday’s attack could potentially prompt more military action.

The attack comes just a few weeks before leaders of more than 100 countries are due to arrive in Paris for high-level climate talks. They were already going to encounter a city on edge. Now, it will be something much darker.

Slate