Collecting Evidence At A Crime Scene
Collecting Evidence At A Crime Scene
Richard Saferstein (Former Chief Forensic Scientist) gives expert video advice on: What is a 'mobile crime laboratory'?; What physical evidence is important for an investigator to collect at a crime scene?; What evidence should a forensic scientist collect from the body of a murder victim? and more...
What is a 'mobile crime laboratory'?
A mobile crime lab, that's a real misnomer. Many police departments today have purchased vehicles that are sent to crime scenes, and they give them the name 'crime labs' or 'mobile crime labs', when in actuality they are really nothing more than vehicles that carry equipment that allows for the documentation of the crime scene. Photographs, sketching equipment, lights and finger printing equipment if any finger printing must be done at the crime scene. So these so called 'mobile crime labs' are basically vehicles that are designed to carry equipment that allows for the proper collection and documentation of evidence at the crime scene. But you know what, I know many a police department that has the contents of a so called 'mobile crime lab' in the trunk of their cars.
What physical evidence is important for an investigator to collect at a crime scene?
That's an interesting question. Obviously, that depends on the crime scene. I mean, you come to a scene, there may be a gun, there may be a bullet, and there may be a knife. Those are the obvious things that you will go for. Then there are the less obvious things, the things that are hidden from sight and you get them by conducting a systematic search. You know, looking very carefully at the scene, looking for things that aren't very obvious. But you know what? There are also things that you can never find at the crime scene. We call them "carriers" of physical evidence. There's perhaps a glass that the lips of the perpetrator touched and as a result, left his or her DNA on. Or, a cigarette butt where a person's DNA may have been deposited. There are objects that have possibly fingerprints on them. They have to be brought back to the laboratory. So we call these items "carriers". We may not know that they're appropriate until we bring them back to the lab and we look at them. So they have to be collected in a very thorough fashion as well.
What evidence should a forensic scientist collect from the body of a murder victim?
The first thing that has to be done is to identify that victim. And that is done through the photographs of the body and finger printing the body. But once that is accomplished, the search for physical evidence becomes the ultimate goal. So, we must do it right the first time. We must collect the clothing of the victim to make sure that there is nothing deposited on that clothing that ultimately has forensic value. Of course, there is that we would carry out in the crime laboratory. Finger nails scraping must be collected from the victim to make sure that there is no small bits of fibers or other items for physical evidence like the cloth in the finger nails of the victim, perhaps is the result of the physical encounter with the perpetrator. If there are sexual overtones to the crime, head and pubic hair swabbings or collections, I should say, must be undertaken. Anal, vaginal and oral swabs must be collected again if there are sexual overtones. In case there was a shooting and there were bullets involve, the bullets must be removed from the body and collected, preserved and sent to the crime lab for analysis. But the important thing is that all of this has to be done the first time around. Once the body is on the ground, once the body's cremated it's pretty difficult to correct for errors.
What is 'substrate control' in CSI?
Dr. Richard Saferstein: 'Substrate control' is an interesting concept because one of the things that we're most concerned about in Forensics is contamination. That is, what we collect at the crime scene and what we find in the crime lab, is that ultimately something that we can relate to the perpetrator, to the victim? Or, is it something that came about as a result in inadvertent contamination let's say by a police officer who happens to be investigating that crime scene or a bystander? So in certain situations, a 'substrate control' is taken of areas that we think are uncontaminated, that are near important areas of the crime scene that contain important items of physical evidence. And we examine these 'substrate controls' to make sure that they are free of contaminants. So when we do examine the evidence that we've collected and the other 'substrate control' and we have information that we can extract from that piece of evidence, we know that that information is free of contamination, free of contaminants.
What is a 'standard reference sample' in CSI?
I spoke earlier about what a crime lab does in terms of examining physical evidence. And one of the things it does is to compare physical evidence. To compare physical evidence for the purpose of ultimately determining whether two or more objects have a common origin. So for example, if you have a hair at a crime scene, you have to have something to compare it against. Normally, hopefully, it will be hair taken from the suspects head. But it could be hair also that came from the victim. So a thorough collection of physical evidence at the crime scene must not only relate to what is present at the scene in terms of commission of the crime, but it must also relate to taking things from individuals that ultimately could be used for comparison purposes. Things such as fingerprints, hair, and samples of, biological samples, that allow for us to determine that persons DNA. This is ultimately carried out for the purpose of conducting the comparison.
What is the 'chain of custody' of CSI?
When you go to court as a forensic scientist, one of the things that you're almost always confronted with is the issue of chain of custody. Many of the participants in the court case don't have much of a background in science, but they do have thorough knowledge about this concept of chain of custody. How do you know the evidence that you're testifying about? How do you know that the evidence that was examined in the crime lab was the item that actually was collected at the crime scene? It has to be documented. The flow of the evidence from the scene to the lab, to the courtroom, must be documented. Essentially, chain of custody means accountability and being able to know where that evidence was at all times, from the moment it was found to the moment it's being testified to. Also, at the same time knowing that it wasn't contaminated, and knowing that its integrity was not impugned in any way.
How does a crime lab study in CSI?
A crime lab basically examines physical evidence for either one of two purposes: either to identify the physical evidence as "What is it?" Is it white powder? Is it heroin? Is it cocaine? That is what we would call the identification or the "What is it?" The other function of the crime lab study is to compare physical evidence to determine whether two or more objects emanate from the same source. Now, for example, if a fingerprint is left at a crime scene and we have a suspect and one of the prints on that suspect's fingers compares to the print left at the crime scene, we have made a comparison and we have determined that that individual and no other individual was present at the crime scene. We call that "individuality", and the ability to compare and to link a person or a thing to a crime scene to the exclusion of all other persons or things.