Tool marks and impressions

When arriving at the scene of a crime where it has been deemed that someone broke into a building it is important to make a clear record of those marks. A record will help to identify what kind of tool was used and, if a suspect is found a similar tool could be found in their possession when a lawful search is conducted. Whenever possible, the evidence in question should be brought to the lab for investigation. If that is not possible, several photos to scale should be taken of the area. After photographing the area, a silicone cast should be taken of all tool marks. The reason for using silicone instead of any other product is because of the clarity it gives the cast.

            After the item or the cast is taken to the lab for testing we can determine the tool that made the impressions. For example, if the tool marks showed class characteristics of a screwdriver then the investigative team will know to look for screwdrivers when conducting a search of a suspects belongings. If said screwdriver is found then it can be examined and compared to the tool marks in the cast. All tools have their own individual characteristic marks and if they have a high probability of matching we can determine that the subject likely used that tool to break into the building. Any crime scene investigator should know that if it appears they may have found the tool that was used they should NEVER press that tool into the area in question. If that tool is pressed into the area then the area will take on the characteristics of the tool when they may not have been there previously. One should never assume that just because there is a gun next to a gunshot victim that the gun is the same one that killed the victim. This is practicing objectivity when examining a crime scene.

            There are other types of impressions that can be found at a scene. For example, there may have been a break-in at the scene but a perpetrator had to have arrived on the scene in a vehicle or by foot. Either way, it is very important to photograph the impressions, to scale, from several angles and with the best contrast. After photographing, cast impressions should be taken of the tire or shoe impressions.  According to our textbook, the following is the recommended procedure for casting impressions:

1.      “Retrieve any fragments of debris that is not imbedded within the impression. Photograph the impression before and after retrieving debris; include a ruler in the photograph. A frame for containing the dental stone may be installed around an impression that is shallow or on an inclined surface.

2.      To solidify the soil, a fixative such as hair spray is used by holding the can about eighteen inches from the soil within the impression. Very lightly spray an even layer to the impression using a sweeping motion and taking care to avoid any damage to the impression.

3.      Wait ten minutes for the hairspray to dry.

4.      Add an appropriate amount of water to a premeasured amount of dental stone, usually ten to twelve fluid ounces of water to about 1.5 to 2 pounds of dental stone in a zip-lock bag.

5.      Pour the dental stone through an opening in the bag onto the ground beside the impression and allow it to run into the impression. Carefully disperse the stone into the impression so that it does not destroy the fine details of the impression. Continue pouring until the dental stone completely fills the impression to .5 to 1 inch in thickness. Label the wet plaster surface with the date, initials and any other required information for evidence.

6.      ‘When the cast no longer adheres to the soul and is relatively dry, remove the cast.”

(Saferstien, Richard. Forensic Science, From the Crime Scene to the Crime Lab. 2013 Pg 222)

If the impressions need to be shipped one should wait one to two days to make sure the impression is completely dry. When casting and packing these impressions it is very important to follow SOP and chain of custody.

      Tire and shoe markings are stored in a general file and is updated periodically by the manufactures. These types of markings such as the sole of a shoe are called class characteristics and can help narrow down a list of suspects but in no way does it absolutely exclude other suspects. Now, if the sole of that shoe had hash marks in it because the wearer was bored and carved on their shoe and the impressions also showed these hash marks then these would be considered individual characteristics and police would be free to pursue this suspect as their perpetrator. If it was determined that the victim was also bitten in the assault, several photos should be taken to document the bite so the court can issue a demand for a dental impression and it can be compared to the bite mark photographs for a match. Careful evidence collection is an important key to prosecuting a crime. The more careful the collection the better chance investigators have of solving the crime.

(Saferstien, Richard. Forensic Science, From the Crime Scene to the Crime Lab. 2013)