How Crime Scene Investigation Works

Crime scene investigation is the meeting point of science, logic and law. "Processing a crime scene" is a long, tedious process that involves purposeful documentation of the conditions at the scene and the collection of any physical evidence that coul­d possibly illuminate what happened and point to who did it. There is no typical crime scene, there is no typical body of evidence and there is no typical investigative approach.

At any given crime scene, a CSI might collect dried blood from a windowpane -- without letting his arm brush the glass in case there are any latent fingerprints there, lift hair off a victim's jacket using tweezers so he doesn't disturb the fabric enough to shake off any of the white powder (which may or may not be cocaine) in the folds of the sleeve, and use a sledge hammer to break through a wall that seems to be the point of origin for a terrible smell.

Who's at the Scene?

- Police officers are typically the first to arrive at a crime scene. They arrest the perpetrator is he's still there and call for an ambulance if necessary. They are responsible for securing the scene so no evidence is destroyed.
- The CSI unit documents the crime scene in detail and collects any physical evidence.
- The district attorney is often present to help determine if the investigators require any search warrants to proceed and obtain those warrants from a judge.
- The medical examiner (if a homicide) may or may not be present to determine a preliminary cause of death.
- Specialists (entomologists, forensic scientists, forensic psychologists) may be called in if the evidence requires expert analysis.
- Detectives interview witnesses and consult with the CSI unit. They investigate the crime by following leads provided by witnesses and physical evidence.

All the while, the physical evidence itself is only part of the equation. The ultimate goal is the conviction of the perpetrator of the crime. So while the CSI scrapes off the dried blood without smearing any prints, lifts several hairs without disturbing any trace evidence and smashes through a wall in the living room, he's considering all of the necessary steps to preserve the evidence in its current form, what the lab can do with this evidence in order to reconstruct the crime or identify the criminal, and the legal issues involved in making sure this evidence is admissible in court.

The investigation of a crime scene begins when the CSI unit receives a call from the police officers or detectives on the scene. The overall system works something like this:

Every CSI unit handles the division between field work and lab work differently. What goes on at the crime scene is called crime scene investigation (or crime scene analysis), and what goes on in the laboratory is called forensic science. Not all CSIs are forensic scientists. Some CSIs only work in the field -- they collect the evidence and then pass it to the forensics lab. In this case, the CSI must still possess a good understanding of forensic science in order to recognize the specific value of various types of evidence in the field. But in many cases, these jobs overlap.

1. At the Crime Scene: Scene Recognition

When ­a CSI arrives at a crime scene, he doesn't just jump in and start recovering evidence. The goal of the scene recognition stage is to gain an understanding of what this particular investigation will entail and develop a systematic approach to finding and collecting evidence. At this point, the CSI is only using his eyes, ears, nose, some paper and a pen.

The first step is to define the extent of the crime scene. If the crime is a homicide, and there is a single victim who was killed in his home, the crime scene might be the house and the immediate vicinity outside. Does it also include any cars in the driveway? Is there a blood trail down the street? If so, the crime scene might be the entire neighborhood. Securing the crime scene -- and any other areas that might later turn out to be part of the crime scene -- is crucial. A CSI really only gets one chance to perform a thorough, untainted search -- furniture will be moved, rain will wash away evidence, detectives will touch things in subsequent searches, and evidence will be corrupted.

Usually, the first police officers on the scene secure the core area -- the most obvious parts of the crime scene where most of the evidence is concentrated. When the CSI arrives, he will block off an area larger than the core crime scene because it's easier to decrease the size of a crime scene than to increase it -- press vans and onlookers may be crunching through the area the CSI later determines is part of the crime scene. Securing the scene involves creating a physical barrier using crime scene tape or other obstacles like police officers, police cars or sawhorses, and removing all unnecessary personnel from the scene. A CSI might establish a "safe area" just beyond the crime scene where investigators can rest and discuss issues without worrying about destroying evidence.

Once the CSI defines the crime scene and makes sure it is secure, the next step is to get the district attorney involved, because if anyone could possibly have an expectation of privacy in any portion of the crime scene, the CSI needs search warrants. The evidence a CSI recovers is of little value if it's not admissible in court. A good CSI errs on the side of caution and seldom searches a scene without a warrant.

With a search warrant on the books, the CSI begins a walk-through of the crime scene. He follows a pre-determined path that is likely to contain the least amount of evidence that would be destroyed by walking through it. During this initial walk-through, he takes immediate note of details that will change with time: What's the weather like? What time of day of day is it? He describes any notable smells (gas? decomposition?), sounds (water dripping? smoke alarm beeping?), and anything that seems to be out of place or missing. Is there a chair pushed up against a door? Is the bed missing pillows? This is also the time to identify any potential hazards, like a gas leak or an agitated dog guarding the body, and address those immediately.

The CSI calls in any specialists or additional tools he thinks he'll need based on particular types of evidence he sees during the recognition stage. A t-shirt stuck in a tree in the victim's front yard may require the delivery of a scissor lift to the scene. Evidence such as blood spatter on the ceiling or maggot activity on the corpse requires specialists to analyze it at the scene. It's hard to deliver a section of the ceiling to the lab for blood spatter analysis, and maggot activity changes with each passing minute. Mr. Clayton happens to be an expert in blood spatter analysis, so he would perform this task in addition to his role as crime scene investigator.

During this time, the CSI talks to the first responders to see if they touched anything and gather any additional information that might be helpful in determining a plan of attack. If detectives on the scene have begun witness interviews, they may offer details that point the CSI to a particular room of the house or type of evidence. Was the victim yelling at someone on the phone a half-hour before the police arrived? If so, the Caller ID unit is a good piece of evidence. If an upstairs neighbor heard a struggle and then the sound of water running, this could indicate a clean-up attempt, and the CSI knows to look for signs of blood in the bathroom or kitchen. Most CSIs, including Mr. Clayton, do not talk to witnesses. Mr. Clayton is a crime scene investigator and a forensic scientist -- he has no training in proper interview techniques. Mr. Clayton deals with the physical evidence alone and turns to the detectives on the scene for any useful witness accounts.

The CSI uses the information he gathers during scene recognition to develop a logical approach to this particular crime scene. There is no cookie-cutter approach to crime scene investigation. As Mr. Clayton explains, the approach to a crime scene involving 13 deaths in a high school (Mr. Clayton was one of the CSIs who processed Columbine High School after the shootings there) and the approach to a crime scene involving a person who was raped in a car are vastly different. Once the CSI has formed a plan of attack to gather all of the evidence that could be relevant to this particular crime, the next step is to fully document every aspect of the scene in a way that makes it possible for people who weren't there to reconstruct it. This is the scene-documentation stage.

Source: science.howstuffworks.com

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