Police officers in Germany have written a special program called Bundestrojaner, which has been hacking Telegram for 2 years now and has access to the right accounts.
Fight fire with fire
With the help of the self-written software, German policemen get access to the correspondence of criminals, who then arrest them for conceived or preparing crimes. For example, in 2015, the German criminal police cracked a group chat of 8 local ultranationalists who were about to set fire to a hostel with emigrants. The police arrested all participants in the correspondence.
The Bundestrojaner program is simple, convenient and easy to break the accounts of Telegram users, but even it is not perfect. So, the police software can not crack encrypted chats, but this function must be turned on manually, therefore most of the existing groups in Telegram are not protected by encryption.