Mathias Pogba has been detained over an alleged plot as part of a gang to extort money from his brother Paul.
Four other people were put in front of a judge on Saturday and have also been placed under investigation, according to Reuters.
Mathias’ lawyer, Yassine Bouzrou, said his client is innocent while Mathias has denied allegations against him in statements on social media.
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"We are going to challenge this decision,” Bouzrou told French broadcaster BFM TV.
In July, Paul Pogba claimed he was the target of a £11.3m blackmail plot.
On August 31, Mathias said that he would reveal great “revelations” about Paul Pogba.
Other social media videos also promised to reveal "very important things" about PSG striker Kylian Mbappe and questions of "integrity" about Paul Pogba's agent Rafaela Pimenta
French prosecutors opened a judicial investigation earlier this month after former Manchester United player Paul Pogba said he was the target of extortion and threats from an organised gang.
Mbappe has sided with Paul Pogba over accusations the Juventus midfielder used a witch doctor to cast a spell on him.
“Today I prefer to believe the word of my team-mate,” he said.
“It is his word against the word of his brother, I am going to believe my team-mate.
“It’s not a time to put more onto him. I’m pretty detached from it all.”
The 2018 World Cup winner Paul Pogba moved to Juventus - for the second time in his career - this summer after leaving Manchester United on a free transfer.
He is yet to feature for the Turin club and underwent knee surgery this month, making him a doubt for the 2022 World Cup in Qatar later this year.
Mathias Pogba plays football professionally and has been capped by Guinea and played for clubs across Europe including Crewe, Wrexham, Crawley and Partick Thistle.
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