HALL - CARPENTER, CO.
By Edwin T. Scallon, Copyright © 1990, 1995, 2008 All Rights Reserved
ACCIDENT RECONSTRUCTION AND BAC CALCULATION PROGRAMS
TRACE EVIDENCE LOCATED AT SCENE AND AT AUTOPSY
Trace evidence is created when objects make contact. The material is often transferred by heat or induced by contact friction. The importance of trace evidence in criminal investigations was shown by Dr. Edmond Locard in the early 20th Century. Since then, forensic scientists use trace evidence to reconstruct crimes and to describe the people, places, and things involved in them. Studies of homicides published in the forensic science literature show how trace evidence is used to solve crimes. Trace evidence is important in accident investigation, where the movement of one part against another will often leave a tell-tale mark. Such analysis is of great use in forensic engineering.
Examples
Vehicular accident reconstruction relies on some marks to estimate vehicle speed before and during an accident, as well as braking and impact forces. Fabric prints of clothing worn by pedestrians in the paint and/or road grime of the striking vehicle can match a specific vehicle involved in a hit-and-run collision. Such traces are also known as "witness marks", especially in engineering and may be critical in understanding how a product failed. A typical witness mark could be an impact depression which broke a product, especially if that mark can be matched to the product which made the impact such as a hammer or nail. Such marks are also commonly encountered in criminal cases and include bite marks, puncture marks, bullet holes, etc.
Protection
The first preservation is to photograph in situ, and then remove the objects showing key traces, protect them, and analyze under controlled laboratory conditions. Many techniques are used in the protection of trace evidence from criminal investigations, although all must be photographed as soon as possible, and while still in place. Samples may be collected by shaking, brushing, tapping, vacuuming, swabbing and handpicking. Great care may be needed to prevent contamination with other substances (such as natural oil and sweat on the hand of the collector). In some cases, such as with oil or grease, solvent extraction can be used to collect the evidence for analysis. The method used for collection is generally dependent on both the type of evidence and from where or what sort of object it is being collected. Trace Evidence is also found in much smaller amounts at crime scenes.
Analysis
Close-up of broken fuel pipe using optical microscopy
Droplets of human blood. The droplets are round and show no splattering, indicating they impacted relatively slowly, in this case from a height of two feet.
Analysis of trace materials most often begins with a visual examination of the evidence usually involving macrophotography. This is then usually followed by microscopic analysis, of which a number of different types are available depending on the type of material to be analyzed, such as a stereomicroscope, scanning electron microscope (SEM) or comparison microscope. SEM is especially useful because X-ray analysis can be conducted on selected areas of the sample, so is a form of microanalysis. It is useful where chemical residues can show unusual elements present which may indicate a chemical attack of the product. A car accident caused by a diesel fuel leak, for example, showed traces of sulfur on the cracked tube indicative of an attack by sulfuric acid from the battery.
Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope equipped with an energy dispersive spectroscopy. Small amounts of explosives, volatile hydrocarbons, and other chemicals are identified with the use of analytical instruments, such as gas chromatography, mass spectrometry, and infrared spectroscopy, all of which separate out the components of the chemicals.
Similar comments apply to damaged items from an accident scene, but care is needed in ensuring that the sample is not damaged by the testing or sampling for testing. Such nondestructive testing must always be used first before considering destructive methods which involve taking small samples from the item for more detailed tests, such as spectroscopic analysis. Use of all such methods must be done in consultation with other experts and the relevant authorities, such as lawyers on both sides of a case.
Case in point. A 6-1/2 year old boy was struck and killed in front of his home by a motor vehicle. The operator fled the scene. No credible witnesses saw the actual collision between the car and the child. No license plate or even a make or model of the vehicle. Collectively, the witnesses agreed it was an older blue sedan. There were no vehicular parts dislodged from vehicle at the accident scene. There were not skid marks. Attending the autopsy, I noted contact striations on the child's back. The distance between these striations were measured. Also, there appeared to be some bluish type specks near these striations.
I suggested to the pathologist to collect those flakes in a sterile glass tube. The pathologist using a tongue depressor and some sterile mineral oil scraped the child's back and collected several bluish flakes. It was clear from the damages found on the child's body, that the vehicle had suffered some front end damage, probably to the grill.
The FBI material analysis laboratory in Washington, DC, using a spectrometer was able to identify the flakes as paint from a motor vehicle. In fact, there were three layers of paint. The original primer, the original paint when the car was manufactured and a layer of paint when the car was repainted that last layer layer was blue. The lab was also able to identify the original paint for this vehicle and gave me a range of dates of manufacture and the manufacturer using that specific paint.
I was looking for a 1968-1974 Oldsmobile. Knowing from the witness statements that this was not a new vehicle I concentrated on the older versions. I contracted Oldsmobile and obtained the grill specification for these vehicles which had a very wide range of models. It turns out however, only one had a grill that had the exact dimensions of the striations measured from the child's back during autopsy. Now I was looking for a 1969 Oldsmobile Cutlass.
I broadcast to all the local departments asking information if they had towed a blue 1969 Cutlass. As it turns out one of the departments towed an abandoned 1969 Oldsmobile Cutlass registered to John Doe (I changed the name for this its not relevant).
I learned that this individual was a friend of a female working in a bar. I went to the bar and located the girl. As it turns out she was this guy's girlfriend. Not identifying myself, I asked if she saw him or knew where he was. The barmaid girlfriend was hesitant to say anything since she did not know me. So, I suggested that I go to John's girlfriend's house, since that was probably where he was. Well I guess that didn't sit well with his barmaid girlfriend so she asked about his other girlfriend. I made up a story and at the end, she said, ". . . after the break-in where he and his partner stole the computers, they took off and hit a kid so he's been hanging low at his sister's house."
I got the sister's name and located her house. After staking this house out for some time, myself and my partner finally approached the sister. I told her I was looking for her brother John regarding a hit and run homicide. I knew she would probably warn him that we were looking for him. When we left, I called the Police Department which towed the 1969 Oldsmobile Cutlass and learned it was impounded in their city at a tow firm who towed for the police. We drove to that lot and we saw a guy smashing the font grill out of a blue Oldsmobile that had prior front end damage. I presented the case to the state wide grand jury and obtained a true bill of indictment. We executed the arrest warrant at his mother's house at 3AM. At court, he plead to the crime(s) and received a ten year sentence.
This is just an example of where and how trace evidence is important. Another story about clothing should be of interest.
When a pedestrian is struck by a vehicle and the operator flees the scene the scene is processed for skid marks and any debris associated with the collision. Rescue responds and transports the victim to the hospital. At the hospital all of the victims clothes are removed so the physician and other medical personnel can begin treating their patient. The clothing is VERY IMPORTANT to the investigation. It is the first item that came in contact with the motor vehicle.
In a subsequent case a old homeless street woman was crossing at an intersection, oddly enough in a cross walk, and a vehicle struck her. She was in critical condition. I processed the scene and responded to the hospital. At the hospital, I prepared a chain of custody form for the clothing. I then noticed that the inside of the long overcoat the woman was wearing had a very unique corduroy ribbing on the lining. A closer look at the lining, I observe yellow paint transfer.
Through some information I developed from "chop shops" I learned that a guy had purchased parts to rebuild a front fender of a yellow Honda. I further learned that this guy is friends with a body shop. Myself along with my partner went to the body shop and right in the bay was a yellow Honda with left front fender damage. I asked the guy working on the car if he knew why were were there and he said, "Yea, the accident with the lady". Naturally, after arresting him and towing the vehicle to the police garage, I dusted the crumpled fender with special fingerprint powder, which under ultraviolet light would show up any contact with the lining of the coat. I turned off all the lights in the garage, turned on the black light and photographed the distinct rib patterns. After photographing the fender, using a very large amount of clear fingerprint tape, I removed the latent imprint which was developed with the powder. I was able to match over 85 distinct patterns between the ribs on the coat material with the imprint left on the fender of the hit and tun vehicle.
These are just two examples of where trace evidence can be found and how important it is for an investigation.