Two attackers stormed a village church during Mass in northern France Tuesday. The pair took hostages and slit the throat of an 85-year-old priest. (James McAuley and Jason Aldag / The Washington Post)

Two attackers backing the Islamic State — including one on a watch list — stormed a village church in northern France during Mass on Tuesday, taking hostages and slitting the throat of an 85-year-old priest before police commandos shot and killed the assailants, authorities said.

A nun who was inside the church told a French television station that the priest was forced to his knees and killed when he attempted to resist. Officials said the attackers screamed “Allahu akbar” (God is great) as they slit his throat.

The slaying came less than two weeks after carnage in the south of France: at least 84 people killed by a truck that plowed through a Bastille Day celebration in the Riviera city of Nice. It also underscored the huge challenges for European security forces as Islamic State-inspired violence increasingly reaches beyond major urban centers and takes the form of local, even intimate, violence.

The Islamic State’s Amaq news agency described the attackers as “soldiers” of the militant network, according to the SITE Intelligence Group, which monitors terrorist activity. But it was not immediately clear whether the assailants had direct contact with the Islamic State.

French authorities identified one of the attackers Tuesday evening as Adel Kermiche, a 19-year-old local resident already known to anti-terrorist investigators and whose parents had reportedly alerted authorities about his radicalization.

An undated image of the Rev. Jacques Hamel, who was slain Tuesday in an apparent Islamic State-inspired attack on his church in Saint-Etienne-du-Rouvray, France. (AP/Diocese of Rouen via AP)

According to Paris prosecutor François Molins, Kermiche was arrested twice in 2015 for trying to travel to Syria — once in Germany, where he attempted to use his brother’s identity, and once in Turkey, where he was caught with his cousin’s ID card.

In a news conference, Molins did not name the second attacker, but he added that authorities had detained a 17-year-old Algerian-born suspect, allegedly the younger brother of another man already wanted by police for seeking entry into Syria.

According to Molins, Kermiche was detained for nearly a year before being released under judicial supervision earlier this year, which required him to wear an electronic bracelet and check in regularly with police. He was permitted to leave his home for four hours a day on weekdays and six hours on Saturdays.

Molins said the church attack began at 9:24 a.m. Tuesday, during Kermiche’s unsupervised leave, when his electronic bracelet was apparently deactivated.

Another person held by the hostage-takers at the church suffered life-threatening injuries, said Interior Ministry spokesman Pierre-Henry Brandet.

“They recorded themselves,” a nun identified as Sister Danielle told BFM-TV as she described the attacks. “They did a sort of sermon around the altar in Arabic.”

The Rev. Alexandre Joly, 44, said the slain priest — identified by church officials as Jacques Hamel — was “like an attentive grandfather” in the village.

French President François Hollande — who traveled to the church near the city of Rouen in the Normandy region — said the attackers had pledged allegiance to the Islamic State.

Hollande’s predecessor, Nicolas Sarkozy, posted a Twitter message saying the bloodshed struck at the “soul of France.”

The assault is likely to put further pressure on European security officials, who joined counterparts from about 40 nations last week in Washington in efforts to expand the battle against the Islamic State and undercut its recruitment propaganda.

After the Nice attack, the Islamic State also claimed a connection with two attackers in Germany who wounded 20 people this month. Two other attacks in Germany that killed 10 people in the past week had no evident connection with the group.

Although Tuesday’s assault was on a smaller scale than three major terrorist attacks that have killed more than 220 people in France since January 2015, it appeared likely to exacerbate criticism of French counterterrorism efforts and elicit more scrutiny of national security practices.

“In my worst nightmares,” said François Heisbourg, a former member of a French presidential commission on defense and national security, “I didn’t imagine that we would actually let loose an individual on furlough who would not be tracked by legitimate intelligence-gathering while he was condemned for terror-related activities with well-established radical credentials.”

Once suspects pass from French police supervision into the country’s judicial system, he said, they are relatively unsupervised, as the French Justice Ministry lacks the security resources of the Interior Ministry.

“Here’s a guy who was let loose for hours a day in his home town,” Heisbourg added. “But nobody can follow him, because the Ministry of Justice doesn’t have a security service, except in prisons.”

Authorities in Europe had previously concentrated on suspected militants who returned from Islamic State-held territory in Syria or Iraq. But many recent attacks appear to have been carried out by individuals radicalized by Islamic State propaganda and who pledged loyalty on their own.

In Saint-Etienne-du-Rouvray, a working-class town about 35 miles southeast of the port of Le Havre, on the banks of the Seine as it snakes toward the English Channel, French soldiers patrolled narrow streets of rowhouses and empty storefronts Tuesday as mourners gathered in the main square. Police cordoned off the area around the church, whose main entrance is an arched wooden door under a stone tower capped by a peaked roof.

The area also reflects the multicultural side of France. A mosque in a nearby town was built for Muslims in the region, including families that moved from North Africa decades ago.

Fatima, 58, an Algerian-born woman who said she had lived in the village for 40 years, joined the crowds in the main square to honor what she called “the memory of the priest.”

“We are with him,” said Fatima, who gave only her first name. “All Muslims are with him.”

Dominique Patin, 52, often went to Hamel’s church since moving to the village 17 years ago. “He was a simple man, a foundation of our community,” she said.

“Why this? Why us?” she asked. “It makes me sick to see it.”

France remains under an extended state of emergency after the truck rampage in Nice, which also injured more than 300 people.

Hollande said the country needed to use “all means” against the Islamic State, but he gave no details on possible new crackdowns or expansion of French support to the U.S.-led coalition conducting airstrikes against the militants’ strongholds in Iraq and Syria.

A Vatican spokesman said Pope Francis was shaken by “the pain and horror of this absurd violence” and “condemned, in the most radical way, any form of hate.”

Murphy reported from Washington. William Branigin in Washington contributed to this report.