Criminal investigation is a litigation activity in which public security organs and people's procuratorates collect evidence, prove the crime of fan, apprehend the criminal, and take necessary compulsory measures against the suspect of fan crime in accordance with legal procedures for cases that have already been investigated. The investigation activities include questioning suspects of crimes, questioning witnesses, conducting inspections, searches, seizing physical evidence, documentary evidence, audio-visual materials, confiscating and freezing deposits and remittances, identifying fugitives, and taking necessary coercive measures against suspects of crimes. For criminal cases that have already been investigated, the investigation personnel must strictly follow the legal procedures, conduct investigations, comprehensively and objectively collect and retrieve evidence materials that prove the guilt or innocence of the suspect, and examine and verify them. Various investigation methods and measures must be taken as needed, but strictly in accordance with legal conditions and procedures.