A man who killed a history teacher in France last week had been in contact with a parent leading an online campaign against the teacher, the investigation into the attack has revealed.
The breakthrough in the case came as President Emmanuel Macron promised more pressure on Islamist extremism after days of clampdowns which have resulted in over a dozen arrests, a mosque ordered shut and a pro-Hamas group to be dissolved.
"Our fellow citizens expect actions," Mr Macron said during a visit to a Paris suburb. "These actions will be stepped up."
47-year-old Samuel Paty was beheaded on his way home from the secondary school where he taught in Conflans-Sainte-Honorine, 40kms (25 miles) northwest of Paris on Friday.
Police sources said the 18-year-old killer had exchanged messages on WhatsApp with the man who wanted Samuel Paty fired after his daughter told him how the teacher had shown cartoons of the Prophet Mohammed in a lesson on free speech.
The parent - the father of a girl in Mr Paty's class - was behind an online campaign urging "mobilisation" against the teacher.
The man, now in police custody, had placed his phone number on Facebook and exchanged messages with the killer, Chechen Abdullakh Anzorov, on WhatsApp in the days leading up to the murder.
Mr Anzorov was shot dead by police soon after the killing.
A photo of the teacher and a message confessing to the murder was found on his mobile phone.
Police have arrested 16 people in connection with the killing, including and four members of Mr Anzorov's family.
Five pupils suspected of accepting payment for pointing Mr Paty out to his killer are among those being detained.
The father who spearheaded the online campaign had published a video protesting against Mr Paty's choice of lesson material.
The mosque, now targeted for a six-month closure, shared this video on its own Facebook page.
President Macron said that a pro-Hamas group active in France - "Cheikh Yassine Collective" - would be dissolved for being "directly implicated" in the murder of the teacher.
The group's founder and Islamist radical, Abdelhakim Sefrioui, is being held by police for publishing a video on YouTube insulting Mr Paty.
French Prime Minister Jean Castex told MPs today that the government was targeting "all associations whose complicity with radical Islamism has been established".
Education Minister Jean-Michel Blanquer said that Mr Paty would be posthumously bestowed France's highest order of merit, the Legion of Honour.