There’s a lot involved in conducting any criminal investigation. For this discussion, I've used a homicide for the examples. The steps below are grouped more or less in categories, but may not always be performed in the exact sequence.
     First response. As easy as it may sound, the first step is to locate the scene to be investigated. If a victim is still at the scene of the crime, a detective knows where to begin. If the victim has been removed from the scene (moved to another area after being killed, died after arriving at the hospital, etc.) a detective may have some work to do just to locate where the crime was committed. Once the scene is located, the area must be cordoned off to prevent any outside disturbance. The crime scene should only be accessed by the investigation team and done so in a way that doesn’t disturb any possible evidence. At every step, careful record keeping is absolutely necessary. All evidence must be properly handled and identified and the chain of custody is recorded. Besides the use of photographs, written descriptions should be provided about the victim, crime scene, locations of evidence, etc.
     Removing the victim. Depending on department policy, a victim may or may not be processed for evidence or removed from the scene before the arrival of a medical examiner. However, the victim can be photographed even before the medical examiner arrives. These should include close-ups of wounds and other injuries as well as pictures that show the position of the victim within the larger crime scene.
     Talk to witnesses. If anyone has knowledge of what happened prior to or during the crime, they need to be interviewed. Even if what they say doesn’t sound relevant to the case, the statement may still prove valuable later. Besides writing down what the person observed, it’s important to get the person’s name and contact information in case they need to be interviewed at a later time or are needed to testify in court.
     Processing the scene. A scene should is processed by only one or two trained team members. The more people moving around within the crime scene and handling evidence, the more likely it is evidence can be destroyed. Some evidence is obvious. Weapons, bullet casings, blood trails, tire tracks, shoe prints, drag marks, and many other things are visible and recognizable by even an untrained eye. Other items may not be so obvious either because of their size or because they seem irrelevant. Hairs, fiber, blood that has been cleaned up, other body fluids, etc. fall into the size category. Before moving any item, it should be measured and photographed.
     At the morgue. Although a medical examiner is the only person who can perform an autopsy, there is still evidence to be collected. The M.E. may recover more hairs, fibers, etc that were overlooked at the scene. They may also recover bullets, perform blood screenings, or find other medical information pertinent to the case. The cause and time of death are recorded by the M.E.
     Set a plan. Once all the evidence has been collected and witnesses have been interviewed, it’s time to sit down and formulate a plan of action. If not already known, all efforts need to be made to identify the victim. Knowing the victim allows investigators to learn more about their lifestyles, friends, activities and other things that may help pinpoint the motive for the murder as well as lead to possible suspects. Personnel are assigned to specific functions within the investigation. The size of a team and the duties assigned depend on department policies and procedures. All photographs, evidence, and eyewitness testimony are used to reconstruct the scene. Determining what happened, as closely as possible, can lead investigators to a possible motive and suspects.