The man who beheaded a history teacher in a Parisian suburb asked students to point out his victim before he launched the attack on Friday afternoon.
As more details emerged of the grisly murder, authorities named the teacher as Samuel Paty, 47.
President Emmanuel Macron said on Saturday the country’s fight against Islamic terrorism was “existential” after the teacher was killed in the wake of showing his pupils caricatures of the Prophet Muhammad from French satirical weekly magazine Charlie Hebdo.
Mr Paty taught history and geography at the school in Conflans-Sainte-Honorine, 30 km (20 miles) northwest of Paris.
French police said the suspect was an 18-year-old born in Moscow of Chechen origin. He had been granted a 10-year residency in France as a refugee in March and was armed with a knife and an airsoft gun, which fires plastic pellets.
France’s anti-terrorism prosecutor’s office said that authorities investigating the killing have also arrested nine suspects, including the grandparents, parents and 17-year-old brother of the attacker.
Police said they opened fire after the attacker failed to respond to orders to put down his arms and acted in a threatening manner.
The teacher had received threats after opening a discussion “for a debate” about caricatures about 10 days ago, a police official told the Associated Press. The parent of a student had filed a complaint against the teacher, another police official said.
The French anti-terrorism prosecutor Jean-Francois Ricard said a text claiming responsibility and a photograph of the victim were found on the suspect’s phone. Mr Ricard said the suspect had been seen at the school asking students about the teacher, and the headmaster had received several threatening phone calls.
The man who beheaded the teacher outside the school where he taught had approached pupils in the street and asked them to point out his victim, Mr Ricard said on Saturday.
Speaking at a news conference, Mr Ricard also said that the attacker, after beheading the teacher, had posted a photograph of the teacher’s body on Twitter, accompanied by a message saying he had carried out the killing.
Charlie Hebdo tweeted: “Charlie Hebdo expresses its sense of horror and revolt after a teacher in the line of duty was murdered by a religious fanatic. We express our deepest support to his family, loved ones and all the teachers.
“Intolerance has just crossed a new threshold and seems to stop at nothing to impose its terror on our country. Only the determination of political power and the solidarity of all will defeat this fascist ideology. This filthy act mourns our democracy but must make us more combative than ever to defend our freedom.”
On Saturday, Prime Minister Jean Castex said France will react with the greatest firmness in the wake of the teacher’s beheading.
“Through one of its defenders, it is the Republic which has been struck in the heart by Islamist terrorism,” Mr Castex tweeted.
“In solidarity with its teachers, the State will react with the greatest firmness so that the Republic and its citizens live, free! We will never give up. Never.”
Caroline Fourest, a prominent French feminist journalist who is a former Charlie Hebdo columnist, told BBC Radio 4’s Today: “I was very, very shocked. Not that this murder was a surprise – now we have lived for almost eight years under the permanent threat of terrorist attack.
“We are particularly shocked because this attack targets a teacher. In the history of the French Republic, teachers symbolise a very special figure. The one who gives you hope for unity and fraternity.
“Sometimes we have the feeling that this battle against religious fanaticism is endless but the only secret hope we have is that teachers, with their patience and pedagogy, will be able to block and diffuse hatred and fanaticism in the heart of the next generation by making them understand satirical culture, Charlie Hebdo, and freedom of speech. But yesterday a bit of that hope was beheaded.”
She argued those who “dare to” defend secularism and freedom of speech in the country are under great strain.
Ms Fourest added: “We cannot win a war because it is an asymmetric war, between reason on one side and madness on the other side. You can be patient and precise. You can explain again and again and again.”
Her comments come after two journalists from a film production firm were stabbed with a meat cleaver outside the old offices of Charlie Hebdo three weeks ago.
An 18-year-old man from Pakistan who was arrested for the attack told police he was upset Charlie Hebdo had republished caricatures of the prophet of Islam.
Fourteen people who are suspected of being involved in the 2015 terror attack on Charlie Hebdo, which took place after the paper published caricatures of the prophet of Islam are currently on trial in Paris – with the trial set to go on until November.
In the French National Assembly, the equivalent of parliament, deputies stood up to pay tribute to the teacher.
Additional reporting by agencies