French police have begun questioning three people following the killing of a policewoman at a station in Rambouillet, southwest of Paris, by a suspected Islamist extremist from Tunisia.
The suspect, a 36-year-old Tunisian man, was shot dead at the scene by police.
The three detained after Friday's attack were part of the man's entourage, according to a judicial source.
President Emmanuel Macron said France would "never give in to Islamist terrorism" after the attack at a police station in Rambouillet, a tranquil commuter town about 60 kilometres from Paris, which revived the trauma of a spate of deadly attacks last year.
France's national anti-terrorism prosecutors opened a terror investigation, also involving the DGSI domestic intelligence service, into the murder of a person holding public authority.
A source close to the inquiry told AFP the attacker shouted "Allahu Akbar" (God is Greatest) during the attack.
Chief anti-terror prosecutor Jean-Francois Ricard, who spoke outside the station along with Prime Minister Jean Castex, confirmed "comments made by the assailant" indicated a terror motive.
Unarmed administrative worker
The 49-year-old woman, identified by her first name Stéphanie, was an administrative assistant and mother-of two. She was returning from a lunch break when she was attacked and stabbed in the throat twice. She died of her wounds shortly afterwards.
The attacker, "Jamel G.", was fatally wounded when an officer opened fire on him. He arrived in France illegally in 2009 but had since obtained residency papers, a police source said, adding that he was unknown to security services. He had just moved to Rambouillet.
About 30 police officers wearing balaclavas raided the suspect's home in the town on Friday evening, AFP reporters at the scene said.
At the same time police searched the home of the person who sheltered Jamel G when he first arrived in France, sources close to the inquiry said.
Macron said on Twitter: "In our fight against Islamist terrorism, we will never give in... The nation is by the side of Stéphanie's family, her colleagues and security forces."
Interior Minister Gerald Darmanin, who also visited officers in Rambouillet, said security would be stepped up at stations nationwide.
Several attacks over the last year have reignited concerns about the spread of radical Islam inside France as well as immigration.
Macron's government has introduced legislation to tackle radical Islamist activity in France, a bill that has stirred anger in some Muslim countries.