French investigators have identified the attacker as Khazzani, an official with knowledge of the investigation said, speaking on condition of anonymity because he was not authorized to speak about it. Khazzani lived in Spain until March 2014 in the southern city of Algeciras, where authorities had picked him up for drug trafficking and noted that he attended a mosque with ties to Islamist militancy.

Spanish authorities had notified other European security agencies when Khazzani left Algeciras, and he was added to watch lists in both France and Belgium, according to authorities in both countries. The French designation flagged him for heightened scrutiny when he entered and exited borders. But there are about 5,000 people on the list, far too many to monitor around the clock, security analysts said.

European authorities have long struggled to know when to act on intelligence that they have collected. The three men behind the January attacks in Paris on the Charlie Hebdo newsweekly and a kosher supermarket had been subject to extensive surveillance over concerns they were plotting attacks, but the monitoring was eventually dropped.