Authorities in northern France have launched an inquiry into allegations of violence and racism by members of a far-right group in the city of Lille, after an undercover Al Jazeera investigation exposed Generation Identity (GI) activists carrying out a racist attack on a teenage girl and admitting to a series of other assaults on Muslims and Arabs.

Thierry Pouquet du Haut Jusse, Lille’s public prosecutor, announced the preliminary inquiry into the racist incidents shown in Al Jazeera’s documentary, Generation Hate, late on Wednesday, according to the Voix du Nord newspaper.

The “people who appear in [Al Jazeera’s] report are going to have to explain themselves to police officers in order to ‘define the criminal offences which might lead to prosecutions’,” said Voix du Nord.

The documentary, broadcast on Sunday, showed one GI activist punching a teenage girl in the head several times in Lille’s city centre in January.

The assailant, identified as Remi Falize, in a secretly filmed conversation, afterwards said: “Girl, or no girl, I couldn’t give a f***. They are just Arabs.”

The 30-year-old supermarket worker was later caught on camera saying that if he was diagnosed with a terminal illness, he would go to Lille’s Wazemmes market and “sow carnage” against Muslims.

Falize made the remark at a private bar called the Citadelle, owned by Aurelien Verhassel, head of Lille’s branch of GI, one of Europe’s fastest growing and most prominent far-right movements.

The anti-Muslim group advocates the defence of what it sees as the identity and culture of white Europeans from what it calls the threat of Islam and mass immigration.

The group gained prominence when its activists occupied a mosque in western France in 2012 and attempted to block a snowy mountain pass on the French-Italian border last April.

Secretly filmed footage from the Citadelle and other parts of Lille also show activists boasting about carrying out violent attacks and making Nazi salutes.

Verhassel was also secretly filmed encouraging members of the Citadelle to violence.

The opening of the inquiry on Wednesday came after letters of concern from Martine Aubry, the mayor of Lille, and Miche Lalande, prefect of the Nord County.

Voix du Nord quoted Lalande saying he denounced “racist remarks and violent actions, proved or planned” relayed in the documentary.

In a statement on Tuesday, Aubry called the comments from GI activists “unacceptable”.

“This is entirely reprehensible and is in breach of the law. I have called upon the local authorities so we can find the appropriate measures to put a stop to the activities of these individuals and to shut down the Citadelle, which should have never opened in Lille,” she said.

Verhassel, leader of the Citadelle, told the Voix du Nord on Tuesday there was “nothing against me” in the Al Jazeera documentary.

Reiterating comments made Al Jazeera through his lawyer, Verhassel said the attacks and Nazi salutes filmed in the documentary “do not concern the Citadelle”.

Al Jazeera’s investigation also revealed close ties between GI and Marine Le Pen’s National Front, one of France’s largest political parties.