French prosecutors said Monday that they detained the CEO of Telegram over an investigation opened in July into alleged illegal activities facilitated by the encrypted messaging app, a platform popular among the cryptocurrency community.

Dmitry Agranovsky, 53, the director of the Moscow-based law firm Liptser, Stavitskaya, and Partners, represents Pavel Durov, the Russian-born billionaire and founder of the messaging app Telegram. Durov is accused of allowing criminal abuse of his free speech-oriented social media platform.