How do public security organs investigate criminal cases?

The public security organ may impose criminal detention on an active criminal or a major suspect. Detainees should be questioned within 24 hours after detention. A criminal suspect may hire a lawyer to provide him with legal advice, complaints and accusations after the first interrogation by the investigation organ or from the day when compulsory measures are taken. The entrusted lawyer has the right to know the charges charged by the criminal suspect from the investigation organ, to meet the criminal suspect in custody and to know the relevant information from the criminal suspect. If the public security organ deems it necessary to arrest the detained person, it shall submit it to the people's procuratorate for examination and approval within three days after detention. Under special circumstances, the approval time can be extended by one to four days. For major suspects who commit crimes on the run, commit crimes many times or commit crimes in collusion, the time for submitting for examination and approval may be extended to 30 days. The people's procuratorate shall, within seven days after receiving the approval letter from the public security organ, make a decision on whether to approve or disapprove the arrest. To sum up, when the court finds new criminal clues in the trial of a case, it can hand them over to the public security organ for handling. After the criminal procedure law transfers the case to the public security organ, the public security organ will decide whether to file the case according to whether there are criminal facts and whether it is necessary to investigate the criminal responsibility of the parties. If the conditions for filing a case are met, the public security organ will intervene to carry out the investigation of the case.