The lawyer for the family of beheaded French teacher Samuel Paty expressed her anger on Tuesday over lies spread on social media which led to the murder.

Paty, a secondary school teacher in a town near Paris, was killed last October by a radical Chechen teenager after showing cartoons of the Prophet Mohammed to students during a civics class about free speech.

A 13-year-old schoolgirl has confessed to police that she lied about being in attendance and falsely accused Paty of asking Muslim children to leave the class while he showed the pictures.

Her father, who has been charged in connection with the murder, posted several incendiary videos on Facebook afterwards based on his daughter's testimony which identified Paty.

-Everything in the investigation showed very early that she lied- the Paty family's lawyer Virginie Le Roy told RTL radio on Tuesday.

She said she was "sceptical" of the version of events recounted by the girl who has said she saw herself as a spokesperson for other pupils and wanted to impress her father.

-A spokesperson of what? Of lies, of events that never happened? This explanation does not convince me and makes me rather angry because the facts are serious, they're tragic- Le Roy added.

The killing of Paty shocked France and led to a fresh debate about freedom of speech, the integration of France's large Muslim population, and the role of social media in whipping up hatred.

Paty was murdered in the town of Conflans-Sainte-Honorine by the 18-year-old Muslim extremist from Russia who had seen the online campaign against the teacher mounted by the schoolgirl's father and another man, a known Islamist preacher.

Both of the individuals behind the Facebook videos have been charged with "complicity in murder" over their postings and are awaiting trial in jail, while the schoolgirl has been charged with slander.

The killer was shot dead by police.

A draft new security law being discussed in the French parliament would make it a jailable offence to publish information online about a public servant knowing that doing so could cause them harm.