“He tried to go to Syria,” Mr. Chentouf said. “At the last Ramadan, he said we should all go to Syria. He tried to push propaganda on us.” Until then, Mr. Chentouf said, Mr. Kermiche was a normal teenager, who drank alcohol and smoked cigarettes. He said he last saw Mr. Kermiche on Saturday at a subway station in Rouen. He was dressed in a long robe and had a beard. “He was perfectly calm,” Mr. Chentouf said.  The attack underscored the vulnerability of France, which has sustained three major terrorist attacks in 19 months: an assault on the satirical newspaper Charlie Hebdo and other locations around Paris in January 2015, which killed 17 people; coordinated attacks on a soccer stadium, the Bataclan concert hall, and cafes and restaurants in and around Paris on Nov. 13, which killed 130 people; and the Nice truck attack.  “I understand this feeling of helplessness, but if the French people absorb this truth that it is a long war which will require resilience and resistance, we need to form a block and stay united,” Prime Minister Manuel Valls told the French television channel TF1.  “For months we knew there would be new attacks, and everything is still being done to eradicate this terrorism in Syria and Iraq, and of course in France, but there are hundreds of radicalized people,” he added.  The nation has been concerned about the threat against churches for some time. In April 2015, the authorities arrested Sid Ahmed Ghlam, a 24-year-old Algerian computer science student. He had amassed a trove of weapons in a Paris apartment and was thought to be planning an attack on at least one church. He was a suspect in the killing of a 32-year-old woman, Aurélie Châtelain, whose body was found in a parked car in Villejuif, a Paris suburb.  Mr. Ghlam had been ordered by Abdelhamid Abaaoud, a Belgian militant who went on to help organize the November attacks on Paris, to open fire on a church in Villejuif, according to a report by French antiterrorism police, but the attack was not carried out.  President François Hollande of France says that his country needs to fight the war against Islamic State “by all our means.”  Since the Villejuif plot was foiled, many houses of worship in France, including mosques and synagogues, have been on a heightened state of alert. The country has roughly 45,000 Catholic churches, so protecting them is a very difficult task.  Vincent Fauvel, a spokesman for the French bishops conference, said in a telephone interview that he was not aware of any specific threat against the Église St.-Étienne before the attack. The secretary of the conference had attended several security meetings with the Interior Ministry, Mr. Fauvel added, most recently in June. Mr. Fauvel said there had been no reinforcement of security in churches after the Nice attack.  St.-Étienne-du-Rouvray, a working-class suburb inhabited by many retired chemical and metal workers, was a peaceful community, residents said, with a substantial immigrant population. The parish priest is of Congolese ancestry; the town’s mosque opened in 2000 on land donated by the Catholic parish.  Pascal Quilan, who works at a funeral home near the church, said that around 9:30 a.m.: “I heard several gunshots. Then, loads of police.” He added, referring to Father Hamel: “It’s a huge loss for the town. He was someone with lots of humility. It’s really unimaginable.”  Sister Danielle, a nun who escaped from the church, said of Father Hamel: “He knew what was happening.”  The Rouen unit of the B.R.I., a police team that specializes in major crimes like armed robberies and kidnappings, “arrived extremely quickly and positioned itself around the church,” an Interior Ministry spokesman, Pierre-Henry Brandet, told reporters in Paris.  After the two assailants were shot, a police bomb squad searched the church to make sure it had not been booby-trapped.  At the Vatican, a spokesman, the Rev. Federico Lombardi, said that Pope Francis was horrified at the “barbaric killing” of a priest and issued “the most severe condemnation of all forms of hatred.”  Mr. Hollande spoke to the pope on Tuesday and told him “that when a priest is attacked, it is the whole of France that is hurt, and that all will be done to protect our churches and our places of worship,” the Élysée Palace said in a statement.  In a statement from Krakow, Poland, where the pope is scheduled to travel this week to attend the World Youth Day celebration, Archbishop Dominique Lebrun of Rouen said that he would return home immediately. So did the parish priest, the Rev. Auguste Moanda-Phuati. A memorial Mass was held at Rouen Cathedral, a Gothic landmark, on Tuesday evening as the nation yet again mourned victims of terrorism.  “The Catholic Church has only prayer and brotherhood among men as its weapons,” Archbishop Lebrun said in an appeal for peace. “I leave here hundreds of young people who are truly the future of humanity. I ask them not to give in to the violence, and to become apostles of the civilization of love.”  The attack drew condemnation from across French society. Dalil Boubakeur, the president of the French Council of the Muslim Faith, called the attack “barbaric and criminal” and declared that “Muslims stand together behind the government to defend France and its institutions.” The Representative Council of French Jewish Institutions said that the attack “marks a new stage in the spread of terrorism in France” and that “the authorities and the population must now quickly adapt to this new emergency.”  But the attack also renewed criticism of Mr. Hollande and his Socialist government from his political rivals. “We must be merciless,” Nicolas Sarkozy, Mr. Hollande’s predecessor as president and the leader of the opposition Republicans, said in a statement to reporters. “The legal quibbling, precautions and pretexts for insufficient action are not acceptable.”  Marine Le Pen, the leader of the far-right National Front and who is expected to run for the presidency, said that both major parties had failed the country. “All those who have governed us for 30 years bear an immense responsibility,” she wrote on Twitter. “It’s revolting to watch them bickering!”  Adam Nossiter reported from St.-Étienne-du-Rouvray and Benoît Morenne from Paris. Reporting was contributed by Daphné Anglès, Lilia Blaise and Martin de Bourmont from Paris, Hannah Olivennes from London and Rukmini Callimachi from Stuttgart, Germany.  Get news and analysis from Europe and around the world delivered to your inbox every day with the Today’s Headlines: European Morning newsletter. Sign up here.
President François Hollande said that the Islamic State was behind the assault, the latest in a series that has left Europe stunned, fearful and angry.