Mathias Pogba, the older brother of the France and Juventus footballer Paul Pogba, is reportedly being temporarily detained and questioned by police after the Juventus midfielder said he was the target of extortion attempts and threats.

“Mathias Pogba presented himself to the investigators in the early afternoon and was taken into custody,” a source close to the investigation said on Wednesday. Three other people are also in custody, added a legal source who asked not to be named.

The formal probe follows preliminary investigations and will look into allegations of blackmail by an armed gang, kidnapping and membership of a criminal conspiracy. Paul Pogba has told the Paris prosecution office he had been the target of extortion by childhood friends since March.

The former Manchester United player paid €100,000 euros (£86,460) to that group in the spring, judicial sources told Reuters this month. Mathias Pogba denied the allegations in statements posted on social media. His lawyer reiterated on Friday that he denies any involvement in the alleged extortion.

That denial in a statement issued by his lawyer, Richard Arbib, came after Mathias Pogba published a bizarre video online on 27 August – in French, Italian, English and Spanish – promising “great revelations” about his younger brother.

“Taking into account how the affair has evolved and the huge media attention surrounding it, Mathias Pogba strongly insists he knows nothing about any extortion attempt against his brother, Paul Pogba,” read the lawyer’s statement.

“It is clear that the difficulties the Pogba family are experiencing are the result of outside threats which the justice authorities, who are already involved, will evaluate. Mathias Pogba, who wishes above all to calm the situation with his brother, will from now on only speak with the investigating magistrates, should it be necessary.”

Two investigating magistrates have been appointed to lead the probe, it was announced last week. The investigation is handled jointly by the French police’s anti-gang and central crime units.

On the field, Paul Pogba’s hopes of playing for France at the Qatar World Cup later this year hang in the balance after a knee injury. The midfielder underwent surgery that was described as “successful” earlier this month.