What does the secret investigation letter mean?

Letters from close contacts who assisted in the investigation. Investigation letter is a letter sent by judicial organs to other units to request their assistance in investigation according to the needs of investigation. Close contact investigation refers to close contacts who assist in the investigation. When big data will identify close contacts, the superior disease control department or administrative department will issue an investigation letter. It is a letter from a close contact requesting local assistance in the investigation, and will inform the community to verify the information and take samples for inspection.