The specific targets in Paris were La Défense, the large office and commercial complex that is just to the northwest of Paris, as well as an unidentified Catholic association, said Claude Moniquet, a former French intelligence officer who now works in Belgium and who has been in regular contact with investigators.

Two of the men who took part in the Nov. 13 attacks in and around Paris, and who died in a police raid days later, had also been planning an assault on La Défense, the Paris prosecutor said at the time. The Islamic State, which has claimed responsibility for the Paris and Brussels attacks, had also mentioned an assault in the 18th Arrondissement of Paris in November, but the exact location was never clear and no strikes ever occurred.

La Défense would be both a symbolic target and an important economic one for terrorists. Tens of thousands of people work in the large complex, which includes the office towers of many of France’s major companies, including Areva, Total and Société Générale. Many multinational companies also have substantial offices there.

La Défense is also home to a sprawling indoor shopping mall, as well as a train station for two of the busiest commuter lines in the Paris region.