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The Crime Scene: Tips for How First Responders Can Help Preserve Key Evidence

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By Dr. Dena Weiss, Faculty Member, Criminal Justice at American Military University

First responders to crime scenes are a crucial part of any criminal investigation. The first officer at the scene has many responsibilities including:

  • Establishing the perimeter of the crime scene
  • Determining where the suspect(s) entered and exited
  • Keeping unauthorized people out of the crime scene

The first officer at the scene walks a fine line regarding their actions at a crime scene. Upon arrival, crime scene investigators (CSI’s) would like to be provided a general idea of what occurred, but at the same time they do not want an officer to disrupt the crime scene.

First Steps at a Scene

As soon as the officer has determined the scene is safe and no individuals require medical attention, he/she should focus on keeping bystanders away. First responders should also separate witnesses immediately until detectives arrive to interview them. Officers should also determine the mode of entry and exit of the crime in order to establish an alternative path for law enforcement to enter the scene without disturbing evidence.

Guidelines Regarding Evidence Collection

Generally, the first officer at the scene should not attempt to collect evidence. However, there are some situations that may warrant evidence collection before CSI’s arrive. For example, if a victim has attempted suicide with a firearm, Emergency Medical Services (EMS), or the first officer at the scene, will need to remove the weapon from the victim’s grip for safety reasons and to provide medical care if still possible.

Another situation where the first officer at the scene may need to collect evidence is in severe weather conditions where the crime scene is outside. Severe thunderstorms may threaten to wash away spent bullets and casings if a shooting occurred in the street. Bloodstains will also be lost if some attempt is not made to shield the stains from the rain or swab a sample during a storm.

[Related: Officer Training to Improve Crime Scene Processing]

In these situations, the officer should make every attempt to photograph and document the items’ location before collection. Chain of custody cards should be filled out for all items collected noting the case number, location, time, item description, and officer’s name and badge number. Any actions at the crime scene, including documenting a crime scene log, should be written in a supplemental report.

Crime Scenes are Fragile, be Conscious of Contamination

In the past, contamination issues that CSI’s were most concerned with included:

  • Absence of personal protective equipment (PPE) such as gloves
  • Officers leaving shoeprints in crucial areas of the scene
  • Officers spitting or tossing cigarette butts within the scene
  • Officers handling doors and windows that were a point of entry/exit by suspect(s)
  • Officers handling electronic devices

[Related: Technology Offers New Ways to Lift Latent Prints from Crime Scenes]

With the recent advances in DNA analysis, contamination has become a greater concern. DNA profiles can now be obtained from minute samples of epithelial cells that are sloughed off when an item is touched or rubbed against. Officers must now be more conscientious of what they are touching at the scene and wear gloves at all times. Forensic laboratories are now able to develop DNA profiles from clothing that a suspect has grabbed or worn, textured handles of weapons, and even shoe strings.

[Related:  Law Enforcement Investigation Tools Expand Beyond DNA to Bacteria]

Another concern regarding the preservation of the crime scene is the alteration of electronic devices. Officers should avoid handling and scrolling through computers or cellphone applications. Often times at a homicide or suicide scene, first officers become curious when they see computers and decide to jiggle the mouse to see what is on the screen or even attempt to view the search history. The electronic device may hold key evidence and should only be examined by a trained computer forensics investigator.

Advancements in forensic science have made every aspect of the crime scene more fragile. First officers at a scene need to be extremely conscious of their activity and movement knowing that key evidence could be anywhere.

crime sceneAbout the Author: Dr. Dena Weiss is a full-time professor at American Military University, teaching courses in criminal justice and forensic science. She has been a crime scene investigator for more than 17 years and is currently a fingerprint expert for a central Florida police department. Prior to that position, she was a serologist for the Florida Department of Law Enforcement. Her court experience includes testifying in court cases in over 15 Florida counties in more than 200 federal and circuit court cases. Dena has a bachelor’s degree in chemistry and sociology from Mary Baldwin College and a master’s degree in forensic science from Virginia Commonwealth University. She is currently working on her PhD in business administration with an emphasis in criminal justice. 


Leischen Kranick is a Managing Editor at AMU Edge. She has 15 years of experience writing articles and producing podcasts on topics relevant to law enforcement, fire services, emergency management, private security, and national security.

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