Security was stepped up at police and fire stations across France on Sunday after a knife-wielding French convert to Islam was shot dead after attacking three police officers.
Bertrand Nzohabonayo was killed Saturday after entering a police station in the central town of Joue-les-Tours armed with a knife, seriously wounding two officers - slashing one in the face - and hurting another.
"The investigation is leading towards an attack... motivated by radical Islamist motives," said a source close to the probe, speaking on condition of anonymity.
The assailant, a French national born in Burundi in 1994, cried "Allahu Akbar" ("God is great") during the assault, added the source close to the probe, which is being carried out by anti-terror investigators from the Paris prosecutor's office.
Local prosecutor Jean-Luc Beck said investigators would seek to determine whether "he acted alone or if he acted on orders."
Interior Minister Bernard Cazeneuve, who rushed to the scene of the attack on Saturday, said he had ordered "security measures to be stepped up" for police personnel and firefighters across the country.
Nzohabonayo had previously committed petty offences but was not on a domestic intelligence watch-list although his brother is known for radical positions and once pondered going to Syria, the source said.
On Thursday, Nzohabonayo posted the Islamic State flag as his profile picture on a Facebook page identified as his by several experts on jihadist groups.
But paradoxically, he had also liked a page called "Islamic State in Iraq: Not in my name", for Muslims that "refuse to be associated" with the violence waged by the radical group.
Photos circulating on social networks showed a smiling man with a shaved head and black beard.
One of his former sports teachers said he was a quiet, reserved boy.
"When he arrived at the football club from the Paris region, he was around 16 or 17," said the teacher, who asked not to named.
"He wanted to be the referee, which is unusual at that age. He was devoted to justice."
Several people near his sister's flat in a poor part of town refused to believe the attack was spurred by radical Islamic motives.
"That's not what our town is about. We have managed to install dialogue and understanding between communities," said Ahmed Moussaoui, a retired man who heads up a local association.
Concern over 'lone wolf' attacks
A witness of Saturday's incident at the police station said he saw four officers grab Nzohabonayo to escort him inside while telling him to "calm down", as he started yelling and struggling.
"I saw a large knife, and then a policeman on the ground with his neck full of blood, squirting blood," Sandgy Dumoulin said.
"Then a policewoman had blood on her head, and a third one - he's the one who fired the shots. He fired four shots."
The incident comes as governments around the world brace for so-called "lone wolf" attacks by individuals returning from waging jihad abroad, or who are simply following Islamic State calls for violence in the countries involved in a coalition fighting the IS group.
Last week in Australia, an Iranian-born Islamist with a history of extremism and violence entered a cafe and held people hostage for 16 hours before being killed. Two of the hostages also died.
Last year in France, a recent convert to Islam also stabbed a soldier in the busy Paris commercial complex and transport hub of La Defense.
And the main suspect in the murders of four people at Brussels' Jewish Museum in May, Mehdi Nemmouche, spent more than a year fighting with extremists in Syria.
Authorities in France believe around 1,200 nationals or residents are involved in one way or another in jihadist networks in Iraq and Syria.
AFP contributed to this report.