Forensics For Dummies
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From the moment the first police officer arrives at the crime scene, he follows a strict set of procedural guidelines designed to protect him and everyone else who's present; guard evidence against damage, contamination, or loss; and document everything that occurs at the scene. Following these procedures and maintaining control of the scene until the crime-scene investigators arrive offer the best chance of getting the evidence needed to identify and convict the perpetrator. Failure to follow these directives can result in the crime remaining unsolved or a known perpetrator walking free.

Distinguishing between primary and secondary crime scenes

There may be more to a crime scene than first meets the eye. In fact, more than one crime scene may exist, depending upon how the crime was committed — not to mention where. Crime scenes therefore are considered either primary or secondary. The primary crime scene is where a crime actually occurred. A secondary crime scene is in some way related to the crime but is not where the actual crime took place.

In a bank robbery, for example, the bank is the primary scene, but the get-away car and the thief's hideout are secondary scenes. In the case of a killer who commits a murder in someone's home but transports the victim's body to a river for disposal, the victim's home is the primary scene, and the killer's vehicle and the point along the river where the body was dumped are secondary scenes.

Primary scenes typically yield more usable evidence than do secondary scenes, but not always. Sometimes the only crime-scene investigators have to work with is a secondary scene — the place where a serial killer dumps a victim's body, for example. Under these circumstances, investigators may not know where the actual murder took place and therefore use evidence they find at the secondary scenes to help them identify the killer or locate the primary scene. They may be able to use fibers from an expensive or unusual carpet they found on the victim to identify the manufacturer, the seller, and ultimately a list of buyers or locations where that particular product has been installed. Doing so can greatly narrow the focus of the investigation and lead police to the primary crime scene and the perpetrator.

Arriving at a crime scene

Regardless of whether the first officer to arrive at a crime scene found out about the crime via a phone call to the station, a radio call from a dispatcher, or directly from a concerned person, he or she must make every effort to detain the person who initially reported the crime and not allow that individual access to the crime scene. Anyone who reports a crime may have witnessed the incident or may have seen or heard something suspicious.

However, because the officer has no way of knowing whether the person reporting the crime is a witness or a suspect, allowing the informant access to the crime scene can mean losing or contaminating the evidence. After all, a perpetrator may

  • Believe that reporting the crime makes him or her less likely to be a suspect
  • Attempt to destroy or remove evidence

Neither of these situations is uncommon, so the officer who arrives first needs to approach the crime scene in a logical and organized manner, protecting the evidence and other people who may be there. Otherwise, harm may come to the officer, fellow officers, victims, witnesses, suspects, and even the perpetrator, or evidence may be damaged or destroyed.

The officer who responds first must make personal safety a primary concern and ensure that the perpetrator or perpetrators no longer are present or a threat. Whenever a perpetrator is present, the officer arrests and secures that person. Thereafter, the officer assists any victims who are present, offers first aid as needed, and mobilizes emergency medical services. After these important tasks are completed, the officer begins preserving the crime scene.

Additionally, the officer may need to detain suspects and witnesses and keep them separate to avoid collusion, meaning that the detainees work together to create a story to tell police. However, at this stage, the officer may not know which is which: A witness may turn into a suspect, and a suspect may actually be a useful witness. Furthermore, the officer may have no reason or legal right to detain some witnesses and thus must obtain accurate identification and contact information from each person who leaves the area.

At the heart of crime-scene protection is the principle of exchange. Locard's Exchange Principle states that when any two people come in contact with each other, they exchange or transfer trace materials, such as hair, fibers, and prints. Every person who enters the crime scene can leave behind evidence of their presence, take away crucial trace evidence on their shoes, clothes, or hands, or otherwise damage or alter any evidence that remains. Thus, access to the scene must be restricted immediately and denied to all witnesses and suspects.

Preserving and processing the scene

The size of the area in which a crime occurs varies from scene to scene, and police must be prepared to quickly determine the size and boundaries of a crime scene. This task is not as easy at it seems. A crime scene may be a single room, an entire house, everything on a property, or even a whole neighborhood.

At a minimum, the crime scene includes

  • The exact spot where the offense took place
  • Areas from which the site can be entered, exited, or even escaped
  • Locations of key pieces of evidence — the body in a murder, a safe or cabinet in a burglary, or an entire structure in a suspicious fire

A crime scene can be cordoned off using crime-scene tape, barricades, automobiles, or even by police officers standing guard. Only personnel who are absolutely necessary for processing the scene are allowed in. This restriction often is more difficult to accomplish than you may think. A victim's family members or neighbors may be emotionally unstable and thus difficult to remove from the area. And, of course, members of the press often have clever ways of gaining access to a crime scene, to say nothing of a captain or other high-ranking official trying to push his or her way past a lowly patrol officer who's following orders to keep everyone without a reason for being there away from the scene. Furthermore, you can never underestimate the meanderings of the curious bystander.

After the scene is secured, the first officer to arrive establishes a security log, which basically is a sign-in sheet that must be signed by any and all visitors to the scene. This kind of crowd control helps the investigation in many ways, not the least of which is limiting the number of people who must be examined when stray fingerprints and shoeprints are found. If investigators can be ruled out, the print or prints may point to the perpetrator.

A crime-scene investigator begins by doing a walk-through examination, getting a feel for the scene and organizing an approach to collecting evidence. During this overview, the crime-scene investigator typically doesn't examine any particular pieces of evidence, but rather looks at the big picture before beginning the tedious work of evidence examination and collection.

Documenting the procedure

While a crime scene is being processed, everything that transpires is documented in notes, sketches, photographs, and perhaps even on videotape. This documentation includes not only the scene and the evidence, but also the surrounding area, particularly the perpetrator's possible entry and exit points.

A designated note taker keeps an accurate account of all activities in and around the crime scene. Sometimes a tape recorder is used, and the verbal notes are transcribed later. Regardless of how they're taken, the notes must be detailed, including an overall description of the scene; an accurate list describing what each piece of evidence is; when, where, and by whom it was found; and who transported it to the crime lab. The note taker also identifies and comments on every photo that is taken at the scene.

Photographs of the scene need to be taken as soon as possible so that they show the scene preserved in an unaltered condition. Photos must be taken prior to moving or removing any evidence (or the body, if there is one). Taking several overview images of the area is a good idea, and if the scene happens to be outdoors, pictures of surrounding areas should be taken from multiple angles and points of view. Close-ups of each item of evidence — and, in murder cases, any and all visible injuries to the corpse (while it's still at the crime scene) — are critical. Videotape provides the advantage of including sound so that comments can be added immediately. But whenever video is used, still photos nevertheless need to be taken, because they offer much greater detail resolution.

Photos also are taken of any injured parties, including the suspect. Full-body and close-up shots of any injuries are obtained. Whether these pictures are taken at the scene or at the hospital (or even in the operating room) depends upon the nature of the injuries. Bumps, bruises, and scratches can be photographed at the scene, but photos of injuries such as gunshot and knife wounds probably have to wait until the victim is transported to the hospital.

In photographs where the size of the object or evidence being recorded is important, the photographer includes a point of reference. A ruler is ideal, but another common object — a cigarette pack, ballpoint pen, or a car key, for example — works in a pinch.

Sketches also are extremely important, because they show the relationship of each item of evidence to other items or to the body. Each piece of evidentiary material is mapped, or located by its distance from two fixed points, such as a wall, a lamppost, or a sidewalk. Doing so provides exact graphic coordinates of each item. Sketches made at the scene may be rough, but they need to be accurate. They can be redrawn later for clarity and aesthetics. Several computer programs are available that help generate clear drawings.

Reconstructing the crime scene

After doing an initial walk-through of the crime scene, the investigator begins mentally formulating a hypothesis of the crime, focusing on the likely sequence of events and the locations and positions of everyone present during the crime. Information like the following may be critical in determining the truthfulness of a suspect or the reliability of a witness:

  • Shoeprints may reveal a perpetrator's every step.
  • Fingerprints may indicate the things the perpetrator touched.
  • Tool marks may signify points of entry or where safes or locked cabinets are pried open.
  • Blood spatters, bullet trajectories, the angle and severity of blows and stabs, and the nature of the victim's injuries can reveal the actual and relative positions of the assailant, victim, and anyone else who was present during a crime.
  • The physical changes that take place in a corpse may indicate whether the body was moved several hours after death.

The investigator looks at each piece of physical evidence to find out whether it supports this theory, considering information obtained not only at the scene but also from the crime lab, medical reports of anyone who was injured, and the medical examiner's autopsy examination. Anything that doesn't fit in with or justify the investigator's theory of the crime must be reconciled; otherwise, the theory must change. As a result, the reconstruction of a crime scene is constantly evolving as more evidence is uncovered.

The investigator continually tests the developing crime theory against the evidence and avoids making any assumptions, no matter how logical they may seem. An investigator may logically believe that a piece of evidence ended up where it did because of a suspect's actions, but if the hard evidence doesn't support this belief, the theory must be held suspect.

If a gun is found just outside the rear door of a house where a homicide took place, logic suggests that the assailant dropped the gun while escaping. Although that's certainly a possibility, without solid evidence, ruling out other possibilities may be difficult. For all investigators know, the gun had been tossed there in an attempt to make a domestic homicide look like a murder committed by a burglar whom the victim supposedly caught in the act. Evidence like the spouse's fingerprints on the gun or the victim's blood on the spouse's shoes may, of course, change the theory, but until all evidence in a reconstruction is considered and explained, investigators can't reach any absolute conclusions.

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