France city of Arras on Friday that President Emmanuel Macron condemned as «barbaric Islamic terrorism.»
Interior Minister Gerald Darmanin said France was now on its highest state of alert and that the Arras attack bore a link to events in the Middle East, where Israel is conducting a military offensive to root out Hamas fighters after their deadly rampage.
Only a day earlier, Macron had urged the French to remain united and refrain from bringing the Israel-Hamas conflict home.
Visiting the site of the attack, Macron paid his respects to the dead teacher, whose body still lay under a cover surrounded by a pool of blood.
"(He) stepped in and undoubtedly saved a lot of lives himself," Macron said. «Our choice is made not to give in to terror, not to let anything divide us.»
The suspect, who has been arrested, was a former student of the Lycee Gambetta high school where the attack happened, a police source said.
One of the attackers' brothers was also detained nearby.
The investigation was handed to the anti-terrorism prosecutor's office.
Police could not confirm local media reports that Friday's attacker shouted «Allahu Akbar».
But the attacker was on a state watchlist of people known as a potential security risk, a police source said. The «Fiche S» contains thousands of names and only a small number are actively monitored.
He was described by the police source as a Russian-born Chechen but by some French media as a Russian-born Ingush.
A security source said an elder brother of the alleged assailant was serving time in prison for links to Islamist militant networks and glorification of terrorist acts.
France has been targeted by a series of Islamist attacks over the years, the worst being a simultaneous assault