The Crime Scene
Crime scene contamination usually results through the actions of the personnel at the scene. In general, the greater number of personnel at the scene, the more likely it is that the scene/evidence will be contaminated. Scene personnel can deposit hairs, fibers or trace material from their clothing or destroy latent footwear or fingerprints. Footwear patterns can also be deposited by crime scene personnel or anyone entering the scene. As Professor Locard has taught us, when two objects come in contact with each other they exchange trace evidence. Each time we enter a crime scene, we not only potentially leave trace evidence behind, but also take evidence away from the scene.
Forensic DNA analysis has become an increasingly powerful investigative tool. Analysis of biological fluids and now cells found at crime scenes can, with relatively high confidence, exclude/include a possible suspect and provide a numerical estimate of the similarity of the crime scene and suspect's DNA. The DNA technology being used in crime laboratories around the country can take samples that are very small or degraded and xerox the DNA present to provide a large enough sample to be analyzed. Because of the analyst's ability to xerox very small amounts of DNA from biological evidence, reducing the potential for contamination at crime scenes becomes ever more significant. Single hairs, perspiration and/or saliva inadvertently deposited by an investigator while at a crime scene can now cost valuable time and create the potential for excluding a viable suspect as well as cloud or confuse the interpretations of the physical evidence.
The level of contamination risk to be expected is related to the type of crime scene and corresponding number of individuals who have access to the scene. At a burglary scene, the victim and the officer taking the report may be the only individuals present. In contrast, a typical death scene would usually be visited by the first responder, paramedics, investigators, crime scene examiners, coroner or medical examiners, prosecuting attorneys and possibly supervisors. Family, friends and neighbors of the victim may be present as well. Obviously, due to the higher number of individuals in contact with the scene, the potential for contamination would be significantly higher at a death scene.
Environmental conditions may also play a major role in the contamination of crime scene evidence. Wind, sun, rain, snow and temperature can play key roles in the destruction of the evidence at a crime scene. For instance, if there is blood at an outdoor crime scene and it rains, the blood may become so diluted that testing of the blood becomes impossible. The same would apply if the blood was exposed to the sun on an extremely hot and humid day. The fluid would be decomposed or contaminated by bacteria to a point where further analysis would be impossible or inconclusive at best.
Wind and temperature are also possible mechanisms for contamination. Wind can carry in contaminates or literally blow away evidence. Temperature in the extremes can obviously cause problems by the items containing evidence being "cooked" or "frozen". This applies to outdoor scenes that are unprotected, but can also apply to indoor scenes with no heat or cooling capabilities.
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The first issue that caught my attention is that the murder happened at a football stadium, which is an open facility and therefore the contaminations come from every corner, especially bearing in mind that 40,000 spectators were gathered there. On the other hand, a smaller number is directly connected to the crime scene, 40 of spectators who were involved in the riot. Each of them might have been the murderer and every each of them might have left some traces that contaminate the evidence or been in contact with the victim....