Even with limited knowledge of evidence and its preservation, a police officer who is the first responder to a crime scene can properly protect, preserve, and, in some cases, collect evidence.
The officer who is the first to arrive at a crime scene must appreciate the importance of preventing or controlling any changes in the crime scene. The two critical factors most likely to change the crime scene are people and the weather. The first factor is the most amenable to officer control. First, the officer must ensure that he/she does not introduce change into the crime scene. The patrol car should be parked away from the crime scene, both to prevent impacting evidence left by the suspect and to prevent any suspect still on the scene from observing the officer. Officer and citizen safety are of primary concern when entering a possible crime scene, even if it may mean compromising some evidence. The search for a perpetrator may inevitably involve officers' leaving some of their own trace evidence at the crime scene. While making a search, officers should limit touching objects and places at the scene. When it is clear that the scene poses no danger, officers should cordon off any area of the scene likely to contain evidence from the crime. This can be done with crime-scene tape and the posting of one or two officers at strategic spots. No unauthorized personnel, including police officers, should enter the scene. In securing the scene, officers should be careful to observe and avoid disturbing any possible evidence. The names of possible witnesses should be obtained, but officers should avoid interviewing a witness or suspect at length. This should be left to follow-up investigators. First-responding officers should document in writing every action and movement that they take, keeping in mind that this is likely to be the subject of any examination and cross-examination should a trial occur.