France seeks to uncover Nice attacker's links

French authorities were on Saturday seeking to ascertain if a young Tunisian suspected of killing three people in a knife rampage inside a Nice church had outside help as police arrested a third man who could have been associated with him.

Brahim Issaoui, 21, only arrived in Europe from Tunisia last month and, according to prosecutors, killed the sexton, a Brazilian woman and a French woman in the attack in the Notre-Dame Basilica on Thursday morning.

The attack, which the government has described as an act of "Islamist terror" sent a shockwave across France, a country already on edge after the beheading this month of a teacher who showed a class a cartoon of the prophet Mohammed.

Issaoui was shot by police multiple times and is currently in a grave condition in hospital.

Investigators have been unable to question him and his precise motivations remain unclear.

French police are currently holding three people under arrest for questioning in the investigation, which is focusing on two telephones found on the suspect after the attack.

A first man, 47, was detained on Thursday evening after being seen next to the attacker on surveillance footage the day before the attack.

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

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

Early on Thursday morning, the attacker cut the throat of Nadine Devillers, 60, and the sexton Vincent Loques, 55, inside the Basilica of Notre-Dame de Nice, investigators say.

A Brazilian mother, Simone Barreto Silva, who was stabbed several times, took refuge in a nearby restaurant but died of her wounds there.

Investigators believe Issaoui arrived illegally in Europe on Italy's Mediterranean island of Lampedusa on September 20. He then arrived at the mainland Italian port of Bari on October 9 before coming to Nice just one or two days before the attack.

The attack came with France still in shock over the October 16 beheading by a suspected Islamist radical from Russia's region of Chechnya of teacher Samuel Paty outside Paris.

The teacher had shown a class a cartoon of the prophet Mohammed in the wake of the controversy generated by the reprinting by satirical magazine Charlie Hebdo of the caricatures to mark the beginning of the trial of suspects over the massacre of its staff in January 2015.

Even before that attack, President Emmanuel Macron had promised a tough new campaign against Islamist radicalism nationwide which had aroused controversy and condemnation from Muslims around the world.

Nice's right-wing mayor Christian Estrosi told Europe 1 radio that he wanted to see a change in the constitution in order to "wage war" against what he termed "Islamo-fascism".

Protests erupted Friday in Afghanistan, Pakistan, Bangladesh, Mali, Mauritania and Lebanon, the latest in a string of mass rallies denouncing France.

Foreign Minister Jean-Yves Le Drian warned that French citizens face a security risk "wherever they are" in the wake of the attack, saying alerts had been sent to all French nationals abroad.

In Issaoui's hometown of Sfax in central Tunisia, his family told AFP they struggled to believe he had carried out the attack.

"It's not normal," said Issaoui's brother Yassine.

But relatives said he had turned to religion and isolated himself in the past two years.

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