Three people were arrested after a female police employee was killed outside Paris Friday by an attacker who fled the scene, a judicial source said.
The three held were part of the "entourage" of the assailant, named as 36-year-old Jamel G, the source added.
Searches were still underway at the attacker's home in Rambouillet, west of the capital, where the deadly assault occurred at a police station where the victim worked.
Terror Squad called
French prosectors opened a terror probe on Friday after a woman working for the police was stabbed to death at a police station southwest of Paris by a Tunisian man who was then shot dead by the security forces.
The attack at the station in Rambouillet, a well-heeled usually peaceful commuter town about 60 kilometres (40 miles) from Paris, revived the trauma of a spate of deadly attacks last year in France blamed on radicals.
France's national anti-terrorism prosecutors said they had opened a terror investigation, also involving the DGSI domestic intelligence service, into the murder of a person holding public authority.
The attack took place in the secure entrance area of the station at around 2.20pm (1220 GMT), a police source added, asking not to be named.
The 49-year-old woman, an administrative assistant who was returning from a lunch break, was stabbed in the throat twice and died of her wounds shortly afterwards, the source said.
The attacker was fatally wounded when an officer opened fire on him. The murdered woman was the mother of two children aged 13 and 18.
Prime Minister Jean Castex, who arrived at the scene, denounced on Twitter a "barbaric act of boundless cruelty" against "a hero of daily life."
"I want to tell the police I share their emotion and their outrage," he added.
The assailant, aged 36, 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.
A French police officer was stabbed to death inside her police station Friday, and her attacker shot and killed by officers nearby, authorities said.
The identity and the motive of the assailant were not immediately clear, a national police spokesperson told The Associated Press. The police officer was an administrative employee in the station, the spokesperson said.
The attack took place just inside the police station in Rambouillet, a town southwest of Paris known for a grandiose former royal estate.
Investigators were looking into whether there might have been a link to terrorism, but the anti-terrorism prosecutor's office said it is not currently involved in the investigation.
France has seen deadly attacks against police in the past, including some by extremists.
Prime Minister Jean Castex, Interior Minister Gerald Darmanin and other top officials were heading to the scene of the attack.