As youths search for answers, they are already being dubbed the “Bataclan generation,” as the French daily Liberation put on their front page, and the social and political consequences are likely to reverberate for years and decades to come…

“I’m sad, and I fear,’’ says Ms. Duval. “These are the places we go to. For the first time, we feel like targets. It could have been us.”

The attacks on the satirical magazine Charlie Hebdo were also near here, but in that attack the target was clear: journalists who terrorists say deserved to die because they mocked Islam. Here the targets find no motivation, no easy answers. “This is about people,” says Mr. Nap, “particularly young people.’’

And, perhaps most disconcerting, it’s about people who were having fun. One tweet expressed it poignantly, with the trending hashtag #MonplusbeausouvenirduBataclan, or “My most beautiful memory from the Bataclan.” The poster writes, “We were just the kids in the dark.”