A teacher was killed in a knife attack in a school in the northern France city of Arras on Friday and the investigation was handed to the anti-terrorism prosecutor's office.
The regional Pas-de-Calais authority said the suspected assailant, who also wounded a second teacher and a school security guard in the attack, was arrested.
The suspect was a Russian-born Chechen and former student of the Lycee Gambetta high school where the attack happened, a police source said. He was on a watchlist of people known as a potential security risk in connection to radical Islamism, the police source added.
Police could not confirm local media reports that he shouted "Allahu Akbar". BFM TV reported he was about 20 years old.
"We're all in a state of shock," said philosophy teacher Martin Doussaut, who was chased down by the attacker but managed to escape unharmed after locking himself down in a room.
BFM TV also said the person killed was a French language teacher, while a sports teacher was stabbed and injured.
Pupils were confined to their classrooms, it added.
President Emmanuel Macron was heading to Arras, his office said. In a televised address to the nation on Thursday, Macron urged the French to remain united and refrain from bringing the Israel-Hamas conflict home.
Nothing so far pointed to a link with events in Israel and the Gaza Strip, a second police source told Reuters.
The suspected assailant's brother was also arrested.
La Voix du Nord newspaper said that pupils in all schools in Arras - a town in the desindustrialised, ethnically diverse northern corner of France - were being held in their classrooms for their own safety.
France has been targeted by series of Islamist attacks over the years, the worst being a simultaneous assault by gunmen and suicide bombers on entertainment venues and cafes in Paris in November 2015.
In 2020, a teacher, Samuel Paty, was beheaded by a Chechen teenager who wanted to avenge his use of cartoons mocking the Prophet Mohammad during a class on freedom of expression.
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