These crimes were not written about in the newspapers, not talked about on the radio. Thousands of criminal cases are still classified as "secret". But it is time to penetrate the closed archives. Masterful swindlers, elusive thieves and even serial killers - they also existed in the Soviet era. But there were also those who entered into an "invisible battle" with them. How did detectives work without mobile phones, computers and the most complex examinations? And what criminal secrets were ordered to be forgotten?