MAGNANVILLE, France — The first time Larossi Abballa appeared on the radar of French terrorism investigators, the only act of violence they could pin on him was killing bunnies.
He had joined a small group of men, all bent on waging jihad, on a trip to a snowy forest in northern France five years ago, when he was 19. There, they videotaped themselves slaughtering the rabbits, bought so the men could grow used to the feel of killing.
When he and seven others were later arrested, the authorities found that several of the men had saved the video of the slaughter on their cellphones, alongside footage of soldiers being beheaded, according to French court records. Mr. Abballa was eventually convicted on a terrorism charge and spent more than two years in prison.