The most important aspect of evidence collection and preservation is protecting the crime scene. This is to keep the pertinent evidence uncontaminated until it can be recorded and collected. The successful prosecution of a case can hinge on the state of the physical evidence at the time it is collected. The protection of the scene begins with the arrival of the first police officer at the scene and ends when the scene is released from police custody.

All police departments and sheriff's offices should include intensive training for its personnel on how to properly protect crime scenes. Potentially, any police officer can be put into the position of first responding officer to a crime scene. The first officer on the scene of a crime should approach the scene slowly and methodically. In some eases this is not altogether practical. The first officer may also be involved in arresting an uncooperative suspect or performing life saving measures on an injured victim. In either ease the officer should make mental or written notes (as is practical in each situation) about the condition of the scene as it was upon the officer's arrival and after the scene has been stabilized. The officer should keep notes on the significant times involved in responding to the crime scene (time dispatched to scene, time left for scene, time arrived at scene, time left scene, etc.). An effort must be made to disturb things as little as possible in assessing the situation. Particular attention should be paid to the floor since this is the most common repository for evidence and it poses the greatest potential for contamination. Notes should also be taken if the officer has to alter something in the investigation. Some things the officer should note include: the condition of the doors, windows, and lighting (both natural and manmade); if there are any odors present; if there are any signs of activity; how EMS or fire personnel have altered the scene; anything essential about the suspect (description, statements, physical condition, mental condition, intoxication, etc.); and anything essential about the victim. Once the scene has been stabilized, the scene and any other areas which may yield valuable evidence (driveways, surrounding yards, pathways, etc.) should be roped off to prevent unauthorized people from entering the area and potentially contaminating it. Investigators and other necessary personnel should be contacted and dispatched to the scene, however, under no circumstances should the telephone at the scene be used. Once the officer has secured the scene, he or she could do the following: record witness names and others who may have entered or been at the scene; separate witnesses and suspect(s); do not discuss the events or the crime with witnesses or bystanders or let the witnesses discuss these events; listen attentively but discreetly; and protect evidence which may be in danger of being destroyed. Any actions taken should be reported to the investigators.

Many times the arrival of additional personnel can cause problems in protecting the scene. Only those people responsible for the immediate investigation of the crime, the securing of the crime scene, and the processing of the crime scene should be present. Non-essential police officers, district attorney investigators, federal agents, politicians, etc. should never be allowed into a secured crime scene unless they can add something (other than contamination) to the crime scene investigation. One way to dissuade unnecessary people from entering the crime scene is to have only one entrance/exit into the crime scene. An officer can be placed here with a notebook to take the names of all of the people entering the crime scene. The officer can then inform them that by entering the crime scene they may pose a problem by adding potential contamination, and the reason that the officer is taking their names is in case the crime scene investigators need to collect fingerprints, shoes, fibers, blood, saliva, pulled head hair, and/or pulled pubic hair from all those entering the crime scene. This will sometimes discourage non-essential personnel from entering the crime scene. The officer can also stop unwanted visitors from entering the restricted areas. If extraneous people do have to enter the scene, then make sure that they are escorted by someone who is working the scene. This is to make sure that they will not inadvertently destroy any valuable evidence or leave any worthless evidence.

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