SFAX/PARIS • The family of a man detained for killing three church-goers in France weeks after leaving his home in Tunisia said they are struggling to believe he carried out the attack.

"It's not normal," said Brahim Issaoui's brother, Yassine, incredulous that his sibling was responsible for the attack, which came amid widespread anger among Muslims over comments by French President Emmanuel Macron.

Mr Macron had strongly defended secular values and the right to free speech after a French schoolteacher, who had shown his class cartoons of Prophet Muhammad, was murdered last month in a Paris suburb.

Issaoui, 21, is in serious condition after being shot multiple times by police in the aftermath of last Thursday's brutal knife attack in the southern city of Nice.

Born to a family of modest means in the central Tunisian city of Sfax, Issaoui had turned to religion and isolated himself in the past two years, his relatives told Agence France-Presse.

"He prayed... (and) went from home to work and back, not mixing with others or leaving the house," said his mother, Ms Gamra, crying as she clutched a passport photo of the young man in a white hoodie.

But before that, "he drank alcohol and used drugs. I used to tell him, 'we are poor and you're wasting money?' He would reply, 'if God wills it, he will guide me to the right path, it's my business'," she added.

But Issaoui's recent religiosity did not mean he had become an extremist driven to murder, said his sister, Aida.

"Our brother isn't a killer, he isn't a terrorist, he isn't an extremist," she said. "If it was really him that did this, it's because someone made him."

Tunisia last Thursday strongly condemned the attack and said it would launch an investigation.

The authorities there said the young man had a criminal record related to violence and drugs but nothing linked to terrorism.

One of 11 siblings, Issaoui lived with his parents in a modest house on a potholed road in a working-class neighbourhood near an industrial zone on the outskirts of Sfax, a city 270km south-east of Tunisia's capital Tunis.

Issaoui had dropped out of school and worked as a motorcycle mechanic. Having put some money aside, he opened an unlicensed petrol station.

After failing to reach Europe the first time, he succeeded the second time, reaching Italy and finding work harvesting olives, his brother said, before he made his way to France.

"He said he went to France because it was better for work and there were too many people in Italy," Yassine said.

Meanwhile, a third person was taken into custody in France on Friday in connection with the Nice knife attack, as the government ramps up security efforts against possible militant attacks.

The first detainee, a 47-year-old man, was held on Thursday evening after being seen next to the attacker on surveillance footage the day before the attack.

A second individual, suspected of contacting Issaoui the day before the attack, was held on Friday.

Police said yesterday that the third man, aged 33, was arrested after being present when the home of the second suspect was raided.

REUTERS, AGENCE FRANCE-PRESSE