Three people died, a hostage and two knifemen, in an attack on a church in Normandy Tuesday morning.
It comes after terrorist attacks across Europe, and is the second attack on French soil this month, after the Nice attack on Bastille day.
Here's what we know so far about the church siege.
1. Two knifemen stormed a French church during mass.
Two men entered the church, in Saint-Etienne-du-Rouvray, south of Rouen, Normandy, armed with knives shortly before 10am local time (9am UK time), said French media sources. They held the priest and the four members of the congregation hostage.
2. The knifemen were killed by police.
Officers from the elite anti-terrorist Raid squad were called in to assist local police. A number of shots were heard over a period of around 15 seconds as the incident came to an end around 40 minutes later.
A spokesman for the ministry of the interior, Pierre-Henry Brandet, said: "The two hostage-takers came out of the church armed and it was at that point that they were neutralised by the police."
3. The priest had his throat slashed.
One hostage was killed in the attack, 84-year-old priest Father Jacques Hamel. Born in the Rouen area, Father Hamel was ordained in 1958 and was assistant priest in the parish of St Etienne.
Eulalie Garcia, who works in a beauty parlour on the same road as the church, told reporters that she knew Father Hamel, who had taught her the catechism as a young girl.
"My family has lived here for 35 years and we have always known him," she said. "He was someone who was treasured by the community. He was very discreet and didn't like to draw attention to himself."
4. Another victim is 'between life and death'.
The priest was joined in the church by four members of the congregation, which included two nuns. One of them was severely injured and is said to be "between life and death" with another said to be less severely hurt.
5. It is thought one of the knifemen was known to the authorities.
Mohammed Karabila, president of the Regional Council of the Muslim Faith for Haute-Normandie and head of the local Muslim cultural centre, said "the person that did this odious act is known, and he has been followed by the police for at least a year and a half".
He said the attacker "went to Turkey and security services were alerted after this". He had no information about the second attacker.
Local media reported witnesses said the men shouted "Allahu Akbar" as they came out of the building. One was described as bearded and wearing a Muslim skullcap.
6. ISIS claimed responsibility for the attack.
The claim, which came in a statement published by the IS-affiliated Aamaq news agency, said the attack was carried out by "two soldiers of the Islamic State" and added that it was in response to its calls to target countries of the US-led coalition which is fighting IS.
French newspaper Le Figaro reported the church was suspected to have been on a list of Catholic places of worship in the area around Paris drawn up as possible targets by Sid Ahmed Ghlam, an Algerian student arrested last year on suspicion of murdering a mother-of-one during a botched attempt to attack a church in Villejuif.
7. Francois Hollande declared he will fight the terrorists.
"We are faced with a group, Daesh, that has actually declared war, and we have to fight this war using all means possible. Of course we have to respect the rule of law, because we are a democracy," he said.
He warned: "Today we must be aware that the terrorists will not give up as long as we don't stop them. That's our will and that's what we are doing tirelessly.
"The French people must realise that they are threatened. They are not the only country - Germany is also threatened - but their strength really is in cohesion."
8. Religious leaders shared their sorrow.
In a statement, the Vatican said Pope Francis shares the "sorrow and horror" felt over the incident, adding: "We are particularly struck because this horrible violence has occurred in a church - a sacred place where we pronounce God's love - with the barbaric murder of a priest and worshippers affected."
9. One person has been detained in connection with the attacks.
Raid special intervention forces were searching the church and its perimeter for possible explosives and terrorism investigators were summoned. Anti-terrorist authorities opened an investigation shortly after the attack.
One person has been detained, said French prosecutors.