French police on Saturday shot dead a knife-wielding man who attacked three officers in a police station while shouting "Allahu Akbar" ("God is great").
The man wounded one officer's face at the entrance to the police station in Joue-les-Tours near the central city of Tours and injured two others before he was killed.
Anti-terror investigators of the Paris prosecutor's office have opened an inquiry into the incident for attempted murder and other offences related to terrorism.
The perpetrator was a French national born in Burundi in 1994 who was known to police for common crimes, a source close to the investigation told AFP.
The attacker "shouted 'Allahu Akbar' from the moment he entered until his last breath," the source said.
"It looked like the sort of act called for by Islamic State," the source said.
"The investigation is leading towards an attack… motivated by radical Islamist motives."
The Islamic State group, which has seized control of swathes of Iraq and Syria using brutal violence, has exhorted its followers to mount attacks in the West.
According to a statement by the interior ministry, the assailant was around 20 years of age, and was "killed (by) police officers present using their issued firearms."
Prime Minister Manuel Valls pledged his support for the "seriously injured" officers who were "in a state of shock".
He said the state would deal "severely" with anyone who attacked the police.
Interior Minister Bernard Cazeneuve, who visited the scene, said the police had been subjected to a "brutal attack".
He paid tribute to the "cool-headedness and professionalism shown by the police officers".
He said two of the officers were badly hurt, while the third was lightly injured.
All three were out of danger, Tours' public prosecutor Jean-Luc Beck told AFP.
"According to the early indications of the probe, all elements point to legitimate self-defence," he said.
The attacker was not on any watch-lists maintained by France's main domestic intelligence service, the General Directorate for Internal Security, the source involved in the inquiry said.
But the source noted the assailant's brother was known to security agencies for his radical convictions and had at one point planned to travel to Syria.
Cazeneuve said he had ordered "security measures to be stepped up" for police personnel and firefighters across the country.
– Returning jihadists –
Authorities believe around 1,200 French nationals or residents are involved in jihadist networks in Iraq and Syria.
Like other European countries, France is concerned about the risk of attacks mounted by nationals who return home after fighting alongside extremists in Syria.
The main suspect in the murders of four people at Brussels' Jewish Museum in May, Mehdi Nemmouche, is a Frenchman of Algerian origin who spent more than a year fighting with extremists in Syria.
In 2012, French Islamist Mohammed Merah killed seven people in the southwest city of Toulouse.
He was killed by police after a 32-hour siege in his flat.
A year later, a 22-year-old French Muslim convert stabbed a soldier in the La Defense business district of Paris.