When police officers are called to the scene of a crime, they must follow a series of procedures in order to build a criminal case that will stand up in court. They may be lucky enough to stop a crime in progress or apprehend a suspect immediately, but even when they cannot arrest right at the scene, they must carry out an investigation in order to make an informed decision about who to charge with which crime and how to prove it.
Police detectives and officers work with a range of tools when conducting a criminal investigation, including interviews and interrogations. They also use forensics (the scientific analysis of physical evidence), crime scene photography, the Combined DNA Index System and other database links, public tips and surveillance, and a host of other methods to identify suspects, which they then try to link to specific crimes.
They must document every step of the investigation carefully for court purposes, which is why all of this evidence must be collected in a systematic and rigorous way. The process may take days, weeks or even years to complete.
When they have sufficient evidence, investigators will interview suspects and witnesses in the hope of getting a confession from them. This is a science, and detectives are skilled at understanding how to gain a suspect’s trust and manipulate them into giving up information. However, the police must always respect a suspect’s Miranda rights and constitutional protections.