Mathias Pogba, who has played professional football in several countries but is now without a club, posted a video to Instagram on Sunday in which he speaks four languages (French, Spanish, English and Italian) and reads a prewritten message in which he promised to make "big revelations" about Paul Pogba and his agent and lawyer Rafaela Pimenta, as well as Paris Saint-Germain striker Kylian Mbappe.
Paul Pogba said in a statement on Sunday: "The recent declarations of Mathias Pogba on social media are unfortunately not a surprise. They are coming after threats and attempts of extortion by an organised gang against Paul Pogba."
"This has been referred to the appropriate authorities both in Italy and in France a month ago."
The Paris prosecutor's office said on Monday it opened an investigation earlier this month into attempted organised extortion, which is being handled by anti-corruption police. It would not provide further details involving an ongoing investigation.
Mathias Pogba, 32, is a free agent after leaving French fourth-division side Belfort in April. He has also enjoyed spells at a number of other clubs across Europe.
"The whole world deserves to know certain things so the public can decide if [Paul Pogba] really deserves the admiration, the respect, his place in the France team, if he is a trustful person and worth representing the world's youth, working-class areas and big brands," Mathias Pogba said on Saturday.
He also criticised the "integrity and professionalism" of Pimenta, who he said the Juventus midfielder considers "like a second mother."
Paul Pogba, a World Cup winner with France in 2018, left Manchester United this summer and joined Juventus on a free transfer. In a previous spell at Juve from 2012 to 2016, he won four Serie A titles.
Information from Associated Press was included in this report.