The facts are terrible: on Thursday morning in the French city of Nice, a knife-wielding man shouting "Allahu Akbar" (God is Great) beheaded a woman and killed two other people in a church. He was then shot by the police but survived.
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Police described the scene as a "vision of horror".
The French anti-terrorist prosecutor said the suspect was a 21-year-old Tunisian national who had arrived in France earlier in October.
After visiting Nice, President Macron said: "If we are attacked once again it is for the values which are ours: freedom, for the possibility on our soil to believe freely and not to give in to any spirit of terror."
The background
On October 16, a French teacher, Samuel Paty, was beheaded by an 18-year-old Muslim.
Mr Paty had earlier given a class on "freedom of expression" as part of the French national curriculum.
In the class, he had shown his students a cartoon or cartoons depicting the Prophet Mohammed. They were the cartoons which prompted two brothers to murder 12 people at the offices of the satirical magazine Charlie Hebdo which had published the cartoons five years earlier.
According to some reports, he had told the class what he was going to do and said that Muslim students were free to leave if they might find the images offensive.
There is a dispute about whether he showed pupils a cartoon depicting the Prophet naked.
Some parents of children at the school took great exception. Posts on social media vilifying the teacher intensified. The atmosphere became enflamed.
A week and a half after the freedom-of-speech class, 18-year-old Abdoullakh Abouyedovich Anzorov waited outside the school gates, identified the teacher and followed him.
If we are attacked once again it is for the values which are ours: freedom, for the possibility on our soil to believe freely and not to give in to any spirit of terror.
- President Macron
He then used a twelve-inch knife to behead him. Witnesses told police they heard the killer shout "Allahu Akbar" during the attack.
The murderer was shot dead by the police.
At the scene later, President Macron said: "One of our fellow citizens was assassinated today because he taught pupils freedom of expression, the freedom to believe and not believe."
"This was a cowardly attack on our compatriot. He was the victim of a typical Islamist terrorist attack."
Seven people have been charged in connection with the murder of the teacher, including two of his students who, according to the BBC quoting the prosecutor, were paid to identify him to the assassin.
The killer is alleged to have told the students that he wanted to "hit" and "humiliate" Mr Paty and "make him apologise for showing the cartoon of the Prophet".
Bataclan
Ten months after the Charlie Hebdo murders, Islamist terrorists massacred 131 people at the Bataclan nightclub and other venues in Paris.
In 2016, a Tunisian jihadist drove a cargo truck through the crowds celebrating Bastille Day in Nice, killing 86 people.
The reactions
There has been widespread condemnation of Thursday's beheading, including from the French Council of the Muslim Faith.
But before the murder of the teacher two weeks ago, there had been a fierce social media campaign against him.
And after his killing, President Macron's announcement of a string of measures to defend French society, including French Muslim society, against radical Islam fuelled from abroad was criticised by leaders of Muslim countries.
According to Aljazeera, the Arabic television network, "Turkish President Recep Tayyip Erdogan has criticised Macron, saying the French leader needed 'mental checks' over his attitude towards Islam.
"Across the Muslim world, some leaders have condemned France and Macron, including Saudi Arabia and Iran; while tens of thousands have attended protests in Bangladesh calling for a boycott of French goods."
The bigger issue
The killings shine a spotlight on a dilemma for governments in liberal democracies: what should the limit on religious belief be in a tolerant society?
Some Muslims believe that any person who shows or produces a picture of the prophet Mohammed is committing a blasphemy, punishable by death.
The author Salman Rushdie believed that he had a right to his free-expression when he wrote his novel The Satanic Verses. But the spiritual leader of Iran issued a death sentence against him and Mr Rushdie went into hiding for many years. Some people connected with the publication were murdered.
But the broader issue of religious rights versus secular rights is not confined to Islam.
Some fundamentalist Christians and ultra-Orthodox Jews also have strong beliefs which clash with mainstream opinion, but they rarely impose their religious beliefs on others through murder.
Simon Longstaff, the director of the Ethics Centre, wrote: "Religious freedom is a fundamental human right.
"However, it is only one such right among others. Rights should be coupled with equivalent responsibilities. We are responsible for the way in which we exercise our rights, realising them to the fullest extent possible without violating the rights of others."
"Where the clash is inevitable and irreconcilable then the damage done to competing rights should be the minimum necessary and not a jot more."
In France today, this is easier said than done. What if members of one group are so certain of their beliefs that they will commit murder in the foulest manner?
President Macron and France are wrestling with that question.