The brutal killing of a history teacher in a town outside Paris was described by French President Emmanuel Macron on Friday as a "terrorist attack."
The man was found decapitated in public in the town of Conflans Saint-Honorine, in the Yvelines department north-west of Paris, broadcaster BFMTV reported.
Macron and Prime Minister Jean Castex visited Conflans Saint-Honorine hours after the attack took place, as government officials convened a crisis meeting to discuss the case.
"One of our citizens was assassinated today because he was teaching, because he was teaching pupils freedom of expression," Macron said.
The suspect had been carrying a knife and had threatened police after the incident near a school at around 5 pm, according to broadcaster BFMTV.
He had apparently shouted "Allahu akbar" ("God is greater" in Arabic). There was no official confirmation of these reports.
The suspect was shot and killed by police in the neighbouring town of Eragny.
The national anti-terrorism prosecutor, Jean-Francois Ricard, opened an investigation for "murder in connection with a terrorist enterprise" and "criminal terrorist association," AFP reported, adding that he was due to hold a press conference on Saturday afternoon.
Four members of the suspect's family, including a minor, were later taken into custody, AFP also reported in the early hours of Saturday, citing a judicial source.
The agency said that the suspect had not been formally identified, but cited a judicial source as saying that an identity document found on him indicated he was born in Moscow in 2002.
Cartoons of the prophet Mohammed
The teacher had reportedly shown cartoons of the prophet Mohammed to his class as part of his lessons, several media outlets reported, citing police sources.
"The assassination of a history teacher is an attack on freedom of expression and the values of the Republic," wrote the president of the National Assembly, Richard Ferrand, on Twitter.
Right-wing populist leader Marine Le Pen said of the attack that "Islamism is waging war on us."
The mayor of Eragny, Thibault Humbert, praised "the speed with which police neutralized the individual" on Twitter.
A series of attacks in France, many claimed by the Islamic State terrorist organization, cost more than 230 lives in 2015 and 2016.
Suspects accused of links to the attacks in January 2015, in which gunmen killed 17 people mainly at the offices of satirical magazine Charlie Hebdo and a kosher supermarket, are also currently before the courts in a trial that is expected to run until mid-November.