The legal rights NGO Agora will represent Kashin and Plushev in court, the journalists wrote.
A Moscow court fined Telegram 800,000 rubles ($14,000) for missing a July 19 deadline to provide decoding data for six phone numbers.
Russia’s Postal Service reportedly delivered the FSB’s demand letter to Telegram’s London office two days after the July 19 deadline passed.
Pavel Chikov, the head of Agora — which has also agreed to represent Durov in his legal battle with the FSB — shared the Postal Service’s letter of apology for the unexplained late delivery on his Telegram channel.