Paris(CNN) An anti-terrorism investigation has been launched in France after a female police officer was killed in a knife attack at a police station in a Paris suburb.
Police on the scene quickly intervened and killed the attacker, Prime Minister Jean Castex told journalists on Friday, referring to the incident in Rambouillet as a "terrorist attack."
Castex said that "our determination to fight terrorism in all its shape is unbroken, more than ever."
French anti-terrorist prosecutor Jean-François Ricard said, "The way this crime was committed" and "the words said by the attacker during the event" led the anti-terror office to take on the investigation.
The attacker had scouted the location before launching the stabbing, Castex said. And witnesses heard the attacker shout "Allahu Akhbar," according to a source close to the investigation.
Three people, believed to be members of the attacker's close circle, have been taken into custody, a French judicial source told CNN.
Castex and French Interior Minister Gérald Darmanin said they were on their way to Rambouillet, a town of 25,000 people 35 miles southwest of the capital.
Earlier, Castex shared his condolences with the victim's relatives, saying they "have the support of the whole nation" in a tweet.
"The Republic has just lost one of its heroines, in an infinitely coward barbaric gesture," he added.
Local representative Aurore Bergé tweeted her condolences, saying that "this is a terrible shock. For Rambouillet. For the Yvelines, grieving again," referring to a fatal stabbing of a police commander and his partner, a police administrator, in 2016.
Reacting to the attack, French President Emmanuel Macron said France would not give up the fight against terrorism.
"She was a police officer. Stephanie has been killed in her police station in Rambouillet, on the already wounded land of the Yvelines. The Nation stands by the side of her family, her colleagues, and law enforcement officers. We will not give up the fight against Islamist terrorism," Macron said on Twitter.
The attack comes amid an extensive public debate over France's model of secularism, which has wracked the nation in recent months and is likely to be a key issue in the presidential election next year.
Known as "laïcité" in French, the separation of church and state is the framework of the country's political system and is deeply ingrained in French culture.
French lawmakers have been working on contentious legislation aimed at combating Islamist radicalism and reinforcing the country's principle of laïcité, as President Macron attempts to appeal to right-wing voters.
But secularism is divisive in a country as diverse as France, which is home to 5 million Muslims -- many of whom live in poorer areas and are often marginalized in politics and media. The vast majority of France's Muslims do not support Islamic extremism, yet still face unfair stereotypes, experts say.
Critics say the bill, which includes measures that would ban head scarves and other overt religious clothing or symbols, risks discriminating against Muslims.
The bill is currently being scrutinized by a cross-party parliamentary commission.
The wider debate over laïcité and whether it works in modern, multicultural France has been intensified by recent terror attacks.
On October 29, three people were stabbed to death at a church in the French city of Nice. A 21-year-old Tunisian national was charged in their murder, and President Macron said the country was under attack by "Islamist and terrorist madness."
Prosecutors said the suspect had photos on his phone of the man who beheaded a teacher near Paris 13 days earlier, potentially indicating a common motive.
The middle school teacher, Samuel Paty, was killed by an 18-year-old man of Chechen origin who, before the attack, said in a post on social media that he wanted to punish Paty for showing caricatures of the Prophet Mohammed in his class.
Macron subsequently issued a strong defense of the principle of freedom of speech and said France would not "give up" the caricatures, which were published in satirical magazine Charlie Hebdo in 2012.
The attacks are the latest in a string of violence linked to the cartoon in France over several years. In 2015, 17 people were killed in a terror attack that began in Charlie Hebdo's offices in Paris and carried on for three days.
The caricatures are considered blasphemous in Islam, and Macron's comments triggered widespread anger in many Muslim-majority countries last year.