SAINT-ETIENNE-DU-ROUVRAY, FRANCE—Daesh, also known as ISIS or ISIL, crossed a new threshold Tuesday in its war against the West, as two of its followers targeted a church in Normandy, slitting the throat of an elderly priest celebrating Mass and using hostages as human shields before being shot by police.

It was the extremist group’s first attack against a church in the West, and fulfils long-standing threats against “crusaders” in what the militants paint as a centuries-old battle for power. One of the attackers had tried twice to leave for Syria; the second was not identified.

“To attack a church, to kill a priest, is to profane the republic,” French President François Hollande told the nation after speaking with Pope Francis, who condemned the killing in the strongest terms.

The Rev. Jacques Hamel was celebrating mass for three nuns and two parishioners on a quiet summer morning in Saint-Etienne-du-Rouvray when the attackers burst in and forced the 85-year-old priest to his knees before slicing his throat, according to authorities and a nun who escaped.

The nun described seeing the attackers film themselves and give a sermon in Arabic around the altar before she fled. Paris prosecutor François Molins said the other hostages were used as human shields to block police from entering. One 86-year-old parishioner was wounded.

The two attackers were killed by police as they rushed from the building shouting “Allahu akbar,” Molins said. One had three knives and a fake explosives belt; the other carried a kitchen timer wrapped in aluminum foil and had fake explosives in his backpack.

One of the assailants was identified as Adel Kermiche, a 19-year-old who grew up in the town and tried to travel to Syria twice last year using family members’ identity documents, but was arrested outside France and handed preliminary terrorism charges.

Kermiche was put under house arrest with an electronic surveillance bracelet after a judge overruled prosecutors and agreed to free him, Molins said. However, the bracelet was deactivated for a few hours every morning as part of the surveillance agreement, said a police official familiar with the investigation who spoke on condition of anonymity because he was not authorized to release details of the investigation. Those hours corresponded to the time of Tuesday's attack.

A statement published by the Daesh-affiliated Amaq news agency said Tuesday’s attack was carried out by “two soldiers of the Islamic State” who acted in response to calls to target nations in the U.S.-led coalition fighting the extremist group in Iraq and Syria.

Haras Rafiq, managing director of the Quilliam Foundation, described the attack as a turning point. “What these two people today have done is ... shifted the tactical attack to the attack on Rome ... an attack on Christianity,” he said.

He warned that it could “radicalize people from both sides of the communities. Muslim and non-Muslim.”

As Europe becomes painfully inured to a summer of repeated bloodshed, the extremists are looking for greater ways to shock, Rafiq said. “This is going into a house of God. This is attacking and killing a priest.”

“We’ve been talking about the danger of the global jihadist insurgency. This is what it looks like,” he said.

The increasing speed with which Daesh has claimed responsibility and the growing number of attacks this summer have left Europe alarmed and fearful.

Targeting a church in the rural Normandy heartland resonated with France’s leadership and Christians across Europe. While France is officially secular and church attendance is low, the country has deep Catholic roots. Daesh extremists have urged followers to attack French churches and the group is believed to have planned at least one earlier church attack that was foiled when the assailant shot himself in the leg.

The slain priest had been at the church for the past decade and “was always ready to help,” said Rouen diocese official Philippe Maheut.

“His desire was to spread a message for which he consecrated his life,” Mahut told The Associated Press. “And he certainly didn’t think that consecrating his life would mean for him to die while celebrating mass, which is a message of love.”

A nun who escaped said the priest was forced to the ground before his throat was slit. “They forced him to his knees. He wanted to defend himself. And that’s when the tragedy happened,” said the woman, identified as Sister Danielle, speaking on BFM television.

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She said the attackers filmed themselves. “They did a sort of sermon around the altar, in Arabic. It’s a horror.”

One person, a minor, was arrested in the investigation. Molins said he is believed to be the 16-year-old younger brother of someone wanted by authorities for trying to go to Syria or Iraq in 2015.

Hollande, visiting the scene of Tuesday’s slaying, denounced what he called “a vile terrorist attack” and one more sign that France is at war with Daesh, which has claimed multiple attacks on France over the past year and a half, and two in Germany over the past week.

The pope condemned the attack in the strongest terms. The Vatican spokesman, the Rev. Federico Lombardi, said in a statement that Pope Francis expressed his “pain and horror for this absurd violence, with the strongest condemnation for every form of hatred and prayer for those affected.”

The town’s mayor, Hubert Wulfranc, in tears, denounced the “barbarism” and, breaking down, pleaded, “Let us together be the last to cry.”

A sombre quiet surrounded Saint-Etienne-du-Rouvray, a suburb of the medieval city of Rouen composed of genteel residential neighbourhoods and working-class quarters with massive apartment blocks.

The cluster of towns near Rouen had already been linked to Daesh. A microcell of recruits from the area included a Frenchman seen cutting the throat of a Syrian soldier in a November 2014 video. Maxime Hauchard was among at least four people who met at a local mosque and later left to join the extremists.

The violence appears unlikely to slow soon because Daesh reaps benefits even when attackers have no particular connection to the extremists fighting and losing territory in Iraq and Syria.

“ISIS seeks to recreate the same image that helped it attract thousands of foreign fighters to Syria and Iraq and elsewhere,” said Michael Horowitz, an analyst with the Levantine Group security firm. “The pace of these attacks is aimed at painting ISIS as an omniscient group capable of humiliating the West, and defying expectations.”

Tuesday’s attack renewed fears of social and religious tensions in France. Support is rising for the anti-immigrant far right, and the country’s millions of moderate Muslims fear a backlash — fears Daesh has been feeding with a constant stream of propaganda.

“It’s a shot directly at Western Christianity,” said Daniel Shoenfeld, an analyst with the Soufan Group. “It’s this effort by the Islamic State and their supporters to drive a further wedge between broader Western society and Muslims.”

The Rev. Alexandre Joly, who knew the slain priest, said “If we are afraid, they have won. They must not win. ... We must not enter in the game of fear, of rejection.”

France’s security services are stretched after eight months under a state of emergency imposed following attacks in November in Paris. They’ve been under new strain since an attack in the southern city of Nice on Bastille Day — July 14 — that killed 84 people and was claimed by Daesh.

French authorities increased security at places of worship after attacks in Paris last year, but ensuring constant, blanket security is difficult in a country with a church in every town and village.

Correction:The Associated Press erroneously attributed information that the electronic bracelet worn by one of the attackers was deactivated for a few hours every morning. The information came from a police official familiar with the investigation, not from Paris prosecutor François Molins.

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