Easy way to get rid of all those old pesky KGB-FSB records.
Mitroykin delivered most of the interesting KGB files to the West many years ago.
Based on what I've read, that building was housing the RECORDS from the Defense Ministry's PROCUREMENTS.
That this happened almost the same day that the ICIJ released the list of world leaders who used 'illegal tax havens' is just surely a coincidence.