This story originates with Der Spiegel, a leading newsweekly in Germany and has been advanced by Bellingcat, the open source investigative platform.
Bellingcat has confirmed, using open source data, that the diplomat was the son of the deputy director of FSB’s Second Service and the head of the FSB’s Directorate for Protection of Constitutional Order, Gen. Alexey Zhalo. Car and address registration data from leaked Cronos databases freely available on the internet shows that the diplomat — who also carries Gen. Zhalo’s first name as patronymic — was registered at the same address as the general both in Moscow and, previously, in Rostov where the family hails from.
Source: Russian Diplomat Who Died at Berlin Embassy is Senior Intelligence Figure’s Son – bellingcat
Deep States Guide to the FSB (in Russian)
