Criminal proceedings take place in a series of stages.
Usually, the police are responding to a citizen's complaint that a crime has been committed. Sometimes, the police observe suspicious activity. Once they are called, or see something suspicious, the police investigate, take statements from witnesses, and prepare a report on their findings. At times, they will arrest people during the course of their investigation. At other times, they will complete their report and submit it to the prosecutor's office for evaluation, and a prosecutor will decide whether charges should be filed against any suspects named in the police report.