The man who beheaded a teacher in a street in France waited outside the school and asked pupils to identify his target, anti-terrorism officials say.
The man then posted images on social media of dead victim Samuel Paty, 47, who had shown controversial cartoons of the Prophet Muhammad to his students.
The attacker later fired at police with an airgun before being shot dead.
The number arrested rose to 10 on Saturday, with police investigating possible links to Islamic extremism.
The attack took place at about 17:00 (15:00 GMT) on Friday near the College du Bois d’Aulne, where Mr Paty taught, in the town of Conflans-Sainte-Honorine, some 30km (20 miles) north-west of central Paris.
President Emmanuel Macron said the attack bore all the hallmarks of a terrorist attack and the teacher had been murdered because he “taught freedom of expression”.
A national tribute will be paid to Mr Paty on Wednesday.
What is the latest on the attack?
Details of the attack and investigation were given by the anti-terrorism state prosecutor, Jean-François Ricard.
He named the suspect as Abdoulakh A. – an 18-year-old man, born in Moscow of Chechen origin. He came to France with refugee status as a boy and was unknown to anti-terrorism police.
He lived in the Normandy town of Évreux, about 100km (62 miles) from the murder scene and had no apparent connection with the teacher or the school.
The man had been before courts but only on minor misdemeanour charges.
As police approached him, he fired rounds from an airgun at police, Mr Ricard said.
They returned fire. The suspect tried to get up and was shot again, being hit nine times in all.
A 30cm-long (12in) blade was found close by. (BBC)