FBI Handbook of Crime Scene Forensics (7 page)
Read FBI Handbook of Crime Scene Forensics Online
Authors: Federal Bureau of Investigation
Tags: #Business & Economics, #Infrastructure, #Social Science, #General, #True Crime, #Murder
Safe insulation can be compared to a known source. Examinations of safe insulation sometimes can determine the manufacturer.
Questions concerning safe-insulation evidence should be directed to 703-632-8449.
Follow the evidence submission directions, including
Requesting Evidence Examinations
and
Packaging and Shipping Evidence
.
- Collect safe-insulation samples from damaged areas.
- Safe insulation can adhere to people, clothing, tools, bags, and stolen items and can transfer to vehicles. If possible, submit the evidence to the Laboratory for examiners to remove the debris. Package each item of evidence in a separate paper bag. Do not process tools for latent prints.
- Ship known and questioned debris separately to avoid contamination. Submit known and questioned debris in leakproof containers such as film canisters or plastic pill bottles. Do not use paper or glass containers. Pack to keep lumps intact.
Obliterated serial or identification numbers—including markings on metal, wood, plastic, and fiberglass—often are restorable. Comparisons can be made with suspect dies.
Questions concerning serial-number evidence should be directed to 703-632-8442.
Follow the evidence submission directions, including
Requesting Evidence Examinations
and
Packaging and Shipping Evidence
.
- For large objects, and if possible, remove the section containing the serial number and submit it to the Laboratory.
- If it is not possible to remove the section containing the serial number, make a cast to submit to the Laboratory.
- Use an acrylic-surface replica cast kit. Call the Laboratory at
703-632-8442
regarding the appropriate cast kit. - Different formulas are used in different temperatures. If possible, move the evidence to a warm area.
- Casts will duplicate foreign material in the stamped characters. Clean the area before proceeding. Remove paint and dirt with a solvent such as acetone, gasoline, or paint remover. Use Naval Jelly to remove rust. Use a soft brush. Do not use a wire brush.
- Build a dam around the stamped characters to retain the acrylic liquid while it hardens. Use a soft and pliable dam material such as modeling clay. Ensure there are no voids in the dam.
- Following the instructions in the kit, mix the liquid and powder for one minute and pour the mixture into the dam.
- The acrylic liquid will take 30 minutes to harden. Remove the cast when it is hard. If paint and rust are on the cast, make additional casts and submit the cleanest one to the Laboratory.
- Indicate from where on the object (often a vehicle) the cast was taken.
- Pack the cast to prevent breakage.
- Use an acrylic-surface replica cast kit. Call the Laboratory at
Shoe print or tire tread impressions are routinely left at crime scenes. These impressions are retained on surfaces in two- and three-dimensional forms. Almost all impressions, including partial impressions, have value for forensic comparisons. The examination of detailed shoe print and tire tread impressions often results in the positive identification of the shoes of the suspect(s) or tire(s) from the vehicle(s) of the suspect(s).
General crime scene photographs must be taken to relate the impressions to the crime scene. Examination-quality photographs then must be taken to obtain maximum detail for forensic examination and must include a scale. All impressions must be photographed using both methods.
General Crime Scene Photographs
General crime scene photographs of shoe print or tire tread impressions must include close-range and long-range photographs. ISO 400 color film should be used. The photographs must show the relationship of the impressions to the surrounding area. General crime scene photographs are not suitable for footwear or tire examinations.
Examination-Quality Photographs
Examination-quality photographs must be taken directly over the impressions using a tripod and lighting. A scale must be in every photograph. The purpose of these photographs is to produce a detailed negative that can be enlarged to natural size. Examination-quality photographs must be taken as follows:
- Place a linear scale such as a ruler next to and on the same plane as the impression. Place a label in the picture to correlate the impression with crime scene notes and general photographs.
- Images should be taken using a 35 mm or medium-format film camera. Low-cost digital cameras do not provide sufficient image detail for examination-quality photographs. Use a manual-focus camera. If the shoe print is made from a colored substance (e.g., blood), color film may be preferable to black and white. In most ambient-light situations, use ISO 100 film. Use ISO 200 or 400 film, if necessary.
- Place the camera on a tripod and position it directly over the impression. Adjust the height of the camera, and if possible, use a normal lens (50 mm for a 35 mm camera). Fill the frame with the impression and scale. Position the camera so the film plane is parallel to the impression.
- Set the f-stop on f/16 or f/22 for a greater depth of field.
- Attach an electronic flash with a long extension cord to the camera.
- Block out bright ambient light with a sunscreen to maximize the light from the flash.
- Focus on the bottom of the impression, not on the scale. Take an existing- or reflected-light photograph.
- Position the flash at a very low angle (10-15 degrees) to the impression. This will enhance the detail of the impression. For consistent exposure, hold the flash at least 5-7 feet from the impression. Shoot several exposures, bracketing toward overexposure to obtain maximum image detail. Move the flash two or more angles to the impression.
- Take the exposures, move the light to another position, adjust the sunscreen, and repeat Steps 7 and 8.
Impressions in Snow
Impressions in snow are difficult to photograph because of lack of contrast. First, attempt to photograph the impressions as if in soil. To increase the contrast, lightly spray snow impressions with Snow Print Wax, a material used for casting snow impressions, or with colored spray paint. Hold the spray can at least 2-3 feet from the impression so the force of the aerosol does not damage the impression. Direct a light application of spray at an angle of about 30-45 degrees so the colored paint strikes only the high points of the impression. Highlighted impressions will absorb heat from the sun and must be shielded until photographed and cast to prevent melting.
Whenever possible, submit to the Laboratory the evidence bearing the original impression. If the evidence cannot be submitted to the Laboratory, use the following techniques to recover the evidence.
Casting Three-Dimensional Impressions
Casting a three-dimensional impression in soil, sand, or snow is necessary to capture detail for examination. Dental stone, with a compressive strength of 8,000 psi or greater, must be used for casting all impressions. The compressive strength is listed on the container along with the proper ratio of powder to water used for mixing. Dental stone is available through local dental supply houses. Colored dental stone is preferred. Plaster of paris, modeling plasters, and dental plasters are not sufficiently hard, do not resist abrasion when cleaned, and must not be used.
Store dental stone in resealable plastic bags. An 8- by 12-inch resealable plastic bag can store two pounds of dental stone powder. With premeasured bags, casting impressions at the crime scene involves only adding water. The bag containing the dental stone powder can be used to mix and pour the dental stone.
To make a cast, add the appropriate amount of water to the bag and close the top. Mix the casting material by vigorously massaging it through the bag for 3-5 minutes. Ensure that the material in the corners of the bag is also mixed. After it has been mixed, the material should have the consistency of pancake batter or heavy cream.
If the impressions are numerous or large, it may be necessary to mix larger quantities of dental stone in a bucket or bowl. The dental stone should be added slowly to the water and stirred continuously for 3-5 minutes. After it has been mixed, the material should have the consistency of pancake batter or heavy cream.
Casting material has sufficient weight and volume to erode and destroy detail if it is poured directly on top of the impression. The casting material should be poured on the ground next to the impression, allowing it to flow into the impression. The impression should be filled with casting material until it has overflowed.
If the mixture does not flow easily into all areas of the impression, use a finger or a small stick on the surface to cause the dental stone to flow into the impression. Do not put the stick or finger more than 1/4 inch below the surface of the casting material because it can damage the impression.
Before the cast hardens completely, write the date, collector’s initials, and other identifying information on it. In warm weather, the cast should be left undisturbed for at least 20-30 minutes. In cold weather, the cast should be left undisturbed longer. Casts have been destroyed or damaged when lifted too soon. If the cast is in sand or loose soil, it should lift easily. Casts in mud or clay may require careful treatment and excavation when being removed.
Allow the cast to air-dry for at least 48 hours. Package the cast in paper, not in plastic. An FBI Laboratory examiner must clean the cast.
Lifting Two-Dimensional Impressions
Lifting an impression allows for the transfer of a two-dimensional residue or dust impression to a lifting film. It also allows the impression to be shipped to the Laboratory for photographing and examination.
An electrostatic lifting device lifts footwear impressions from porous and nonporous surfaces without damaging the impressions. This device works on dry dust or residue impressions on clean surfaces but will not work if the impressions were wet or have become wet. Electrostatic lifting devices come with instructions for use.
Lifted impressions are damaged easily if the film is not stored properly. The film has a residual charge that attracts dust and debris and causes the film to cling to other surfaces. To preserve and store the lifting film containing an impression, tape one edge of the film securely in a clean, smooth, high-quality paper file folder or tape the edges securely in a shallow photographic paper box. Low-grade cardboard boxes such as pizza boxes must not be used because the residual charge on the film will pull dust from the box and contaminate the impression.
Items that contain a dry residue footwear impression must not be wrapped or stored in plastic because a partial transfer of the impression to the plastic will occur.
Gelatin lifters can be used to lift impressions from porous and nonporous surfaces. Black gelatin lifters work well for lifting light-colored dry or wet impressions. White gelatin lifters can be used to lift impressions developed with fingerprint powders or impressions dark enough to contrast with a white background.
Adhesive lifters can be used only to lift impressions from smooth, nonporous surfaces. White adhesive lifters can be used to lift impressions developed with fingerprint powders. Transparent adhesive lifters can be used to lift impressions developed with black or fluorescent powders. Transparent tapes such as two-inch fingerprint-lifting tape also can be used to lift powdered impressions if the impressions are transferred to a white card.
- Electrostatic:
can be used on porous and nonporous surfaces. Used to lift dry dust and residue impressions. Nondestructive. Useful for searching for latent impressions. - White adhesive:
can be used on smooth, nonporous surfaces. Used to lift wet or dry impressions that have been chemically enhanced or developed with dark fingerprint powder. - Transparent adhesive:
can be used on smooth, nonporous surfaces. Used to lift wet or dry impressions that have been treated with black or fluorescent fingerprint powder. Do not use on an original impression. - White gelatin:
can be used on all porous and nonporous surfaces as long as the gelatin contrasts with an impression. Used to lift wet or dry impressions that have been chemically enhanced or developed with fluorescent fingerprint powder. - Black gelatin:
can be used on all porous and nonporous surfaces. Used to lift wet or dry impressions. Offers good contact with most residue.
A file of shoe manufacturers’ designs and a file of tire treads and other reference material can be searched to determine brand names and manufacturers.
Questions concerning shoe print and tire tread evidence should be directed to 703-632-7288, 703-632-7314, or 703-632-7315.
Follow the evidence submission directions, including
Requesting Evidence Examinations
and
Packaging and Shipping Evidence
.
- For shoe print and tire tread comparisons, submit original evidence whenever possible (shoes, tires, photographic negatives, casts, lifts).
- For shoe print and tire tread file searches, submit quality photographs of the impressions. If photographs are not available, submit casts, lifts, or the original evidence. Detailed sketches or photocopies are acceptable. Images of impression evidence may be submitted electronically. Call
703-632-7288
for specifics on submitting evidence in this manner. - Unobtrusively write the collector’s initials, dates, and other relevant information on the evidence.
- Air-dry and package evidence separately in Bubble Wrap; clean, smooth, high-quality paper or laminated folders; or paper bags, depending on the items being submitted for examination.
