Two attackers killed a priest with a blade and seriously wounded another hostage in a church in northern France on Tuesday before being shot dead by French police.
The attack took place during morning mass at the Saint-Etienne parish church, south of Rouen in Normandy. Five people were initially taken hostage.
There were no immediate details on the identity or motives of the two assailants but the investigation was handed to the anti-terrorist unit of the Paris prosecutor's office.
A police source said it appeared that the priest had had his throat slit.
The attack is the latest in a string of deadly assaults in Europe, including the mass killing in Nice on Bastille Day and four incidents in Germany. Many of the attacks have had links to Islamist militants.
The Archbishop of Rouen identified the slain priest as Father Jacques Hamel. The Vatican condemned what it said was a "barbarous killing".
French Interior Ministry spokesman Pierre-Henry Brandet told France Info radio that the perpetrators have been killed by France's BRI, its elite police anti-crime force, when they came out of the church.
He said that bomb squad officers aided by sniffer dogs had been scouring the church for any possible explosives.
Prime Minister Manuel Valls branded the attack "barbaric" and said it was a blow to all Catholics and the whole of France.
"We will stand together," Valls said on Twitter.
The attack will heap yet more pressure on President Francois Hollande to regain control of national security, with France already under a state of emergency 10 months ahead of a presidential election.
The Normandy attack came 12 days after a 31-year-old Tunisian Mohamed Lahouaiej Bouhlel plowed his heavy goods truck into a crowd of revelers in the French Riviera city of Nice, killing 84 people. Islamic State claimed that attack.