French police have arrested nine people after a suspected Islamist sympathizer beheaded a school teacher on the street of a Paris suburb on Friday, police sources said.
Investigators were trying to establish whether the attacker, who was shot dead by police, had acted alone or had accomplices.
French media reported that he was an 18-year-old of Chechen origin.
Witnesses heard the assailant shout "Allahu Akbar" or "God is Greatest," a police source said.
His victim, a history teacher, had earlier this month shown his pupils cartoons of the Prophet Mohammad in a lesson on freedom of expression.
President Emmanuel Macron visited the scene on Friday evening and called the incident Islamist terrorism.
"One of our citizens was murdered today because he was teaching, he was teaching students about freedom of expression, the freedom to believe and not believe. Our compatriot was flagrantly attacked, the victim of an Islamist terrorist attack."
Four relatives of the attacker, including a minor, were detained in the immediate hours after the attack in the middle-class suburb about 20 miles north-west of central Paris, police sources said.
Five more were detained overnight, among them two parents of pupils at the school where the teacher was employed.
Muslim leaders condemned the killing, which many public figures perceived as an attack on the essence of French statehood and its values of secularism, freedom of worship and freedom of expression.