The billionaire founder of secure messaging app Telegram was arrested in France at an airport outside of Paris. Pavel Durov, who is CEO of the app, was taken into custody on Saturday evening when his private jet landed at the Bourget airport. He was held as part of a French police investigation into alleged illegal activity on the messaging platform, Reuters reported. Russian-born Durov left Russia for Dubai in 2014 after he refused to comply with Putin’s government demands that he shut down opposition accounts on the platform. The app has gone on to become one of the most popular messaging platforms in Ukraine, Russia and other former Soviet republics. According to news reports, the investigation is focused on Telegram’s lack of moderators, with police alleging that its setup could cause criminal activity to go unchecked. Russia’s embassy in France said it is taking “immediate” steps to look into the situation. Telegram will reportedly hit one billion users in the next year.