The timing in Paris early Friday morning could hardly have been more ominous. As the Mayor of Paris and top French officials were on a flight from Lima, Peru, where the International Olympic Committee had awarded them the right to host the Summer Games in 2024, events were unfolding on the ground.

As the airplane approached Paris, a man wielding a knife lunged at a soldier in the heart of the city, in the heavily used Chatelet Metro station, while expressing support for ISIS, according to police. The timing was almost certainly unintended, and the attacker arrested before he inflicted any injuries. And yet, the assault served as a telling reminder—if any was needed—of a crucial question that will loom over the city ahead of August 2024: Just how safe will the Paris Olympics be? …

‘This return migration (of ISIS foreign fighters) has already started,” Otso Iho, senior analyst of Jane’s Terrorism and Insurgency Center in London told TIME on Thursday. In the years ahead, “they are likely to inject added capabilty to existing Islamic networks, combat experience, small-arms handling, ideological rigor, ability to construct explosives, etc.,” Iho says.

This presents a particular issue for France. About 1,800 French joined jihadist groups in Iraq and Syria over the past five years, the greatest number of any European contingent, according to The Soufan Group, a terrorism research organization. Whether these returnees will still pose a threat in 2024 is impossible to predict. Parisians now have seven years to watch, wait and plan ahead.