(CNN) -- A deadly hostage-taking at a Catholic church in Normandy, in which a priest was killed and another person seriously wounded, was a terror attack committed in the name of ISIS, French President Francois Hollande has said.
Speaking to journalists in the northern French town of Saint-Etienne-du-Rouvray, where two men took five people hostage during morning Mass Tuesday, Hollande said the attack was a "cowardly assassination" carried out by "by two terrorists in the name of Daesh" -- another name for ISIS.
The Catholic priest, the Rev. Jacques Hamel, 86, was killed when two men stormed the church in the northern region of Normandy, Dominique Lebrun, the Archbishop of Rouen, said in a statement posted on the diocese website.
Hamel had his throat slit in the attack, said Agnes Thibault Lecuivre, spokeswoman for Paris' anti-terrorism prosecutor.
A man was arrested Tuesday in connection with the attack, Lecuivre said. He was arrested near the church, she told CNN.
Besides the slain priest, two nuns and two churchgoers were taken hostage, CNN French affiliate BFMTV reported.
Sister Daniele Delafosse was able to escape the attack, she said, according to BFMTV. Before she fled, she told the station that she witnessed the perpetrators, who were filming the attacks, gather around the church altar and perform some sort of religious oration in Arabic before forcing Hamel to his knees and placing a knife to his neck. The congregants pleaded with the attackers to stop, she said.
One of the hostages was seriously wounded, and is "between life and death," French Interior Ministry spokesman Pierre-Henry Brandet told reporters.
The situation ended when the two attackers were shot dead by police, he said. "The two killers came out and they were neutralized," he said.
Hollande: ISIS 'has declared war on us'
The priest's killing comes on the back of a string of violent attacks across Europe in recent days, some claimed by the Sunni terror group ISIS, most notably an attack in the French city of Nice less than two weeks ago that left 84 dead.
France has been under a state of emergency since the Paris terror attacks in November last year.
A French police source told CNN that one of the church attackers had tried to go to fight in Syria last year but had been stopped in Turkey by authorities there.
He was then deported to France and sent to prison in May 2015 before he was released, placed under police surveillance and forced to wear an electronic monitoring tag.
According to a French intelligence source, the attacker tried to enter Syria twice after becoming radicalized following the Charlie Hebdo magazine office attacks in Paris. The attacker was associated with Maxime Hauchard, a French jihadi who appeared in an ISIS beheading video in 2014, the source said.
Trouble keeping track
French authorities have struggled to monitor thousands of domestic Islamic radicals on their radar. In response to the heightened terror threat, Hollande has vowed to double the number of officials charged with the task.
More than 10,000 people are on their "fiche S" list, used to flag radicalized individuals considered a threat to national security.
Speaking to reporters, Hollande said: "Daesh has declared war on us. We have to win that war."
The ISIS-linked Amaq News Agency released a statement Tuesday, posted by the group's supporters, claiming the Normandy attackers were the terror outfit's "soldiers." The statement uses language similar to the wording that Amaq recently adopted following the Nice, France, attacks, the southern Germany stabbings and the suicide attack on the German music festival.
CNN cannot independently confirm the claim, and no evidence has surfaced showing ISIS had been in direct contact with the attackers.
Hollande urged the public to remain unified in the face of the threat.
"All people feel affected so we must have cohesion. ... No one can divide us," he said. "Terrorists will not give up on anything until we stop them."
He expressed his sympathies to Catholics, and also met with special forces personnel who responded to the attack.
Saint-Etienne-du-Rouvray Mayor Hubert Wulfranc later called the attack an "act of raw cruelty" and said he had called the city council together to discuss how the city would pay tribute to the victims.
"I told the president it was urgently necessary for this to never happen again," he said, struggling to speak as he fought back tears. "Together, let's be the last ones to cry; and together, let's be the last ones standing against barbarism in the respect of all."
The Paris anti-terror prosecutor has taken over the investigation into the attack, France's Interior Ministry said in a statement.
Vatican condemns killing
The Vatican has condemned the attack, calling it "terrible news" on the back of a string of recent violent attacks in Europe. It said the Pope had been informed of the attack and shared the pain and horror in response to the "absurd violence."
The statement said the violence was particularly horrific as it had taken place in a church, "a sacred place where the love of God is announced."
Lebrun said in a statement that the "Catholic church cannot take up any other weapons but prayer and brotherhood among men."
He called on the faithful "to lower their arms before violence and to become an apostle of a civilization of love."
Other religious leaders were quick to condemn the violence, with the Archbishop of Canterbury, Justin Welby, tweeting: "Evil attacks the weakest, denies truth (and) love, is defeated through Jesus Christ. Pray for France, for victims, for their communities."
The presidential palace tweeted that Pope Francis and Hollande spoke following the attack. The president told the pontiff "that when a priest is attacked, it is all France that is wounded and that everything would be done to protect its churches and places of worship," according to a statement from Hollande's office.
French PM: 'We will stand together'
French Prime Minister Manuel Valls tweeted his horror at the "barbaric attack" on the church, and vowed a defiant response. "We will stand together," he wrote.
A police cordon has been set up around the scene in the town, about 108 kilometers (67 miles) northwest of Paris. A police raid at a house in Saint-Etienne-du-Rouvray ended with authorities leading a man in handcuffs from the home. Several boxes and files also were taken from the home. The reason for the raid and the identity of the man arrested were not immediately clear.
The wounded hostage was treated at the scene, and the three other hostages freed, he said. Explosives experts are working to check if there are any bombs left at the scene.
A witness, Dominique Michot, told CNN that the hostage situation was underway when he arrived at his nearby workplace shortly before 10 a.m. local time (5 a.m. ET).
Michot, a baker who spoke to CNN from inside the police perimeter, said he heard several rifle bursts at about 10 a.m.