The suspect had been on the government’s terrorism watch list since 2016, placed there, according to French news reports, because of his contacts with a man whose wife had attempted to go to Syria.
The terrorism watch list is broad and long — there are about 20,000 names on it. Successive governments have said that the list is merely a tool to help keep loose track of those in danger of turning to terrorism.
Even slight indications of jihadist affinity, as in the case of the suspect in Saturday’s attack, are enough to land individuals on the list. There are too many to follow closely, officials say, and it does not signal past criminal activity.
After each attack, nevertheless, opposition politicians call for a crackdown, sometimes demanding the internment or expulsion of all those on the list.
“Once again we learn that the terrorist was in the S Files,” said Marine Le Pen, the leader of France’s far-right National Front, referring to the government’s antiterrorism list. “What use are the S Files if we don’t use them to neutralize these time bombs on French soil?”
A government spokesman, Benjamin Griveaux, responded in a broadcast on Sunday that “zero risk doesn’t exist” and that “those who say pulling responses out of a hat will solve the problem are lying.”
On Sunday, the police questioned Mr. Azimov’s parents and searched an apartment connected to the family in the 18th Arrondissement of Paris. He was born in the Russian republic of Chechnya and became a naturalized French citizen in 2010. He grew up in Strasbourg, in eastern France, according to a French judicial official. The official said that a friend of Mr. Azimov’s, also born in 1997, had been taken into custody in Strasbourg.
Apart from being questioned by security services in 2017 over his connections to the Syria departure, he had no previous run-ins with the authorities.