NICE, France (AP) — Mourners lit candles and prayed silently Friday to honor the three people who were killed in a French church by a young Tunisian, as France heightened security nationwide amid religious and geopolitical tensions around published cartoons mocking the prophet of Islam.

Tunisian anti-terrorism authorities opened an investigation Friday into an online claim of responsibility by a person who said the attack was staged by a heretofore-unknown Tunisian extremist group.

Around the Mideast and Asia, Muslims offended by the caricatures of the Prophet Muhammad and French President Emmanuel Macron's firm stance against political Islam held more anti-French protests Friday.

Investigators detained a second suspect in Thursday's attack on the Notre Dame Basilica in the Riviera city of Nice, a judicial official said. The 47-year-old man is believed to have been in contact with the assailant the night before, according to the official, who was not authorized to be publicly named.

The attacker, Ibrahim Issaoui, was seriously wounded by police and hospitalized in life-threatening condition, authorities said. French authorities called the attack “Islamist terrorism,” and prosecutors in France and Tunisia are investigating.

A substitute prosecutor at the Tunisian anti-terrorism prosecutor's office, Mohsen Dali, told The Associated Press that the claim of responsibility came in an online post saying the attack was staged by a group called Al Mehdi of Southern Tunisia, previously unknown to Tunisian authorities.

In Nice, four soldiers with rifles periodically walked past the church Friday as mourners placed flowers, messages and candles at the entrance, crossing themselves and praying silently for the three victims.

They included 55-year-old Vincent Loques, a father of two who was the church's sacristan, in charge of its holy objects, according to local broadcaster France-Bleu. Another was a 44-year-old mother of three from Brazil, according to Brazil's Foreign Ministry. France-Bleu said her name was Simone and she had studied cooking in Nice and helped poor communities in the area.

In an interview broadcast Friday with Saudi-owned Al-Arabiya TV, the attacker's mother said she was shocked by the events.

From the Tunisian province of Sfax, the mother, her eyes wet with tears, said she was surprised to hear her son was in France when he called upon his arrival and had no idea what he was planning. “You don’t know the French language, you don’t know anyone there, you’re going to live alone there, why, why did you go there?” she said she told him over the phone at the time.

His brother told Al-Arabiya that Issaoui had informed the family he would sleep in front of the church, and sent them a photograph showing him at the cathedral where the attack took place. “He didn’t tell me anything,” he said. A neighbor said he knew the assailant when he was a mechanic and held various other odd jobs, and had shown no signs of radicalization.

France’s anti-terrorism prosecutor said the suspect is a Tunisian born in 1999 who reached the Italian island of Lampedusa, a key landing point for migrants crossing in boats from North Africa, on Sept. 20 and traveled to Bari, a port city in southern Italy, on Oct. 9. It is not clear when he arrived in Nice.

Tunisians fleeing a virus-battered economy make up the largest contingent of migrants landing in Italy this year. Italian media reported that from Lampedusa, where Issaoui was one of 1,300 arriving migrants on Sept. 20, he was placed with 800 others on a virus quarantine boat in Puglia.

Italy’s interior minister confirmed Friday that the suspect was ordered to leave Italy on Oct. 9. Minister Luciana Lamorgese did not give further details on what, if any action, was taken to ensure the man complied with the order, but she said he was not flagged by either Tunisian authorities nor by intelligence agencies.

Lamorgese called Thursday’s attack in France ‘’an attack on Europe. Let’s not forget that Lampedusa, Italy is the gateway to Europe.’’

The attack was the third in less than two months that French authorities have attributed to Muslim extremists, including the beheading of a teacher who had shown caricatures of the Prophet Muhammad in class after the images were re-published by satirical newspaper Charlie Hebdo.

The images deeply offended many Muslim s, and protesters burned on French flags, stomped on portraits of Macron or called for boycotts of French products at demonstrations Friday in Pakistan, India and Afghanistan. Other protests, largely organized by Islamists, were held across the region, including in Lebanon and the Gaza Strip.

Macron said he would increase the number of soldiers deployed to protect French schools and religious sites from around 3,000 to 7,000. Schools remain open during a nationwide lockdown that started Friday to stem the spread of the virus, but religious services are canceled.

France’s interior minister said Friday that the country is “at war” with Islamist extremists, and the conservative lawmaker for the Nice region, Eric Ciotti, called for a “French-style Guantanamo” to lock up terrorist suspects.

French Muslims denounced the killings, while warning against stigmatizing the country's peaceful Muslim majority.

Nice imam Otmane Aissaoui decried a “terrible act of terror, of savagery, of human insanity that plunges us into sadness, shock and pain" — and once again puts French Muslims in the spotlight.

The attacker “hit brothers and sisters who were praying to their lord,” he told The Associated Press. “It's as if a mosque was touched ... I am deeply Christian today.”

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Charlton reported from Paris. Colleen Barry contributed from Milan.