Four school students are among 15 people who have been taken into custody in France following the beheading of teacher Samuel Paty, who showed controversial cartoons of the Prophet Muhammad to his pupils.
The other detainees include four family members of the killer, a father at the school and a known Islamist radical, reports the BBC.
Police carried out some 40 raids on the homes of suspected radicals on Monday and more raids are expected.
The brutal killing has shocked France.
Tens of thousands of people took part in rallies across the country on Sunday to honour Mr Paty and defend freedom of speech. A ceremony paying tribute to Mr Paty, who was 47, will be held at the Sorbonne University in Paris on Wednesday.
A man named as 18-year-old Abdoulakh A was shot dead by police on Friday after killing Mr Paty close to his school in Conflans-Sainte-Honorine, a north-west suburb of Paris.
What's the latest from the investigation?
The killer's grandfather, parents and 17-year-old brother were detained shortly after the gruesome attack.
The father of a pupil who reportedly launched an online campaign against Mr Paty and a preacher described by French media as a radical Islamist were among six people arrested on Saturday. Interior Minister Gérald Darmanin accused the two men of having issued a "fatwa" against the teacher.
The dozens of raids on Monday targeting suspected Islamist radicals were not necessarily linked to the investigation into Mr Paty's murder.
But Mr Darmanin said police would be interviewing about 80 people who were believed to have posted messages in support of the killing.
The government also said it was probing 51 French Muslim associations. It said if any were found to be promoting hatred, they would be closed down.
One organisation, the Collective Against Islamophobia in France (CCIF), was labelled an "enemy of the state" by Mr Darmanin.
The organisation, which monitors anti-Muslim hate crime in France, has accused the minister of slander after he claimed the group was "obviously" involved in Friday's attack.
In a statement following news of the attack, the CCIF expressed "our pain and our sadness to the family of this teacher".
Why was Samuel Paty targeted?
Anti-terrorism prosecutor Jean-François Ricard said Mr Paty had been the target of threats since he showed the cartoons of the Prophet Muhammad during a class about freedom of speech earlier in October.
As he had done in similar lessons in recent years, Mr Paty, a history and geography teacher, advised Muslim students to leave the room if they thought they might be offended.
Depictions of the Prophet Muhammad can cause serious offence to Muslims because Islamic tradition explicitly forbids images of Muhammad and Allah (God).
The issue is particularly sensitive in France because of the famous publishing of cartoons of the Prophet Muhammad by satirical magazine Charlie Hebdo. A trial is currently underway over the killing of 12 people by Islamist extremists at the magazine's offices in 2015.
Some French-Muslims say they are frequent targets of racism and discrimination because of their faith - an issue that has long caused tension in the country.
What happened in Friday's attack?
Abdoulakh A is understood to have been an 18-year-old Chechen, born in Moscow, who had been living in the Normandy town of Évreux, about 100km (60 miles) from the murder scene. He had no known links to either Mr Paty or his school.
Mr Ricard said that the killer went to the school in Conflans-Sainte-Honorine on Friday afternoon and asked students to point out the teacher.
The suspect followed Mr Paty as he walked home from work. He then used a knife to attack the teacher, before beheading him.
Witnesses are said to have heard the attacker shout "Allahu Akbar", or "God is Greatest".
When police responded to the scene and confronted him, he fired at them with an airgun. Officers returned fire, hitting him nine times. A 30cm-long (12in) blade was found close by.
Authorities said the man had been before courts in the past but only on minor misdemeanour charges.