A criminal investigation is the process that law enforcement officers undergo to collect evidence of, and information about, a crime to determine the crime's seriousness and apprehend the perpetrator for trial and possible sentencing. Evidence includes things physically left behind by the perpetrator, such as finger- and footprints, and eyewitness reports.
During a preliminary investigation, the lead investigator's main duty is to evaluate the crime scene. He must converse with the first responders on the scene and get any observations or activities that he may have missed. He must determine any safety concerns (for example, issues related to bloodborne pathogens), the boundaries of the crime scene and the necessity of getting a search warrant. Above all, he must document absolutely everything in writing and photographs -- or ensure that someone else does. This allows him to develop a plan for any follow-up investigations, as well as preserve the integrity of the scene itself.
Evidence must be placed in a temporary but secure storage area during a preliminary investigation for protection, transport and comprehensive examination. If there is fragile or perishable evidence that may be compromised, it must be documented thoroughly. During a follow-up investigation, investigators may wish to conduct additional searches and canvassing of the crime scene in search of any missed evidence. Laboratory results of any evidence under examination must be also be reviewed as part of the follow-up investigation.
Suspects, Victims and Witnesses
The preliminary investigation is the time when the investigator and any responding officers on the scene identify and detain any suspects, surviving victims and witnesses that may be at the scene. Initial interviews may also be conducted during the preliminary investigation and suspects may be taken into custody. During the follow-up investigation, the investigator runs background checks on suspects, victims and witnesses, interrogates suspects, and conducts additional information-gathering interviews with witnesses and victims.
Officers and Other Law Enforcement Professionals
Both the preliminary and follow-up investigation processes require an investigator to stay in constant contact with the other colleagues working on the case, like the first responders to the scene, photographers, security and lab personnel. In fact, one of an investigator's major responsibilities during the preliminary investigation is distributing duties amongst officers and determining whether additional personnel or resources are needed for the investigation. During the follow-up investigation, an investigator may need to conduct extensive interviews with the first responders and other officers on the scene.