THE BELOW OF THE SURVEY
Cathie Louvet's chronicle
CRIMINAL INVESTIGATIONS :
Folder n°3: in search of traces
My friend Neville and I now know that perfect crime does not exist; there are always clues that drag on the scene of a crime, whatever it may be. Still need to know how to spot them ... Let's follow a team of real experts
1.To secure :
After having equipped themselves with protective clothing, our experts forbid access to the crime scene by surrounding it with the famous yellow ribbon on which are inscribed the words "Technical and scientific police" if they are police officers or "National Gendarmerie" if they are gendarmes operating. In some cases, the team leader may decide to install screening or to mount an inflatable tent, in particular to protect the area from the elements.
In order not to pollute the crime scene and also to protect itself from possible contamination (for example, in the case of infected blood), technicians wear a very specific suit: a single-use combination, shoe protectors, a headgear, an oral mask, gloves and several pairs of surgants.
The next step is to freeze the scene to facilitate further investigation. To do this, the photographer technician photographs the place in three ways: first, a general view of the place, the approaches and the possible accesses; next, he will make a second series of snaps close to each index; finally, it will execute detail views specified by a millimetric rule placed next to each object. The sooner the scene must be reopened to the public or to the owners (in the case of a private place), the more priority is given to the photographer's mission.
Traces and clues are identified by numbered or lettered riders. Their provision will allow investigators to formulate their first assumptions.
5.Traces papillaires :
This is how the police call fingerprints. Visible to the naked eye on smooth surfaces such as glass, or revealed by various techniques, these traces are a valuable indicator for investigators. In order to highlight them, the technician can use either chemical solutions that react to the sweat contained in the traces, or a magnetic powder that will adhere to greases. Thanks to the "dancer", a small magnetic brush, he recovers the surplus powder, thus showing the contours of the imprint. Once revealed, the trace is photographed or transferred to a form using a transparent adhesive. Whenever possible, he picks it up without destroying it so that the lab can search for the DNA it contains, multiplying the chances of identifying the culprit.
6.Traces of DNA :
The precious DNA can be found in the hair plucked with its bulb, under the nails of the victim if it has scratched its aggressor, but also in the spots of urine, sperm or saliva; without forgetting contact DNA wherever the criminal has touched surfaces. Often invisible to the naked eye, these two last types of traces can nevertheless be revealed thanks to the "Crimescope", a powerful "light box" emitting different light rays, from ultraviolet to yellow. Once the trace has been detected, the technician brings the support to the lab where it takes a swab (a large cotton swab). On surfaces that are difficult to read, matt or rough, for example in the case of
gun, he can try his chance a little randomly by rubbing a moistened swab. It also takes samples from the mouth of the suspect (s).
7.Traces of blood :
Even leached, traces of blood can be detected by our experts thanks to a solution made with two pellets based on luminol mixed with distilled water. Simply spray it on the suspect surface: in contact with the iron contained in the l hemoglobin of the red blood cells, a violet light is emitted, the blood is fresh, dry, old, washed with water, passed through the detergent, and without any damage to the DNA traces that remain exploitable because they are not contained in the red blood cells but in the white blood cells.
8.Raising fingerprints :
Traces of footsteps can sometimes help to specify the size and weight of a suspect; traces of tires prove that such vehicle was at the scene of the crime. Thus, the technician searches them essentially close to the body of the victim and access points. If the trace has been printed in soft soil, it molds, taking care to stabilize fragile materials such as sand or snow with a fixative, sometimes hairspray or wax.
And for the scarcely visible traces, it uses the same developers as for the papillary traces. In the particular case of a 3D imprint found in a carpet or deep carpet, a holographic image can be produced by laser and then printed on a photo film.
9. Establish a site survey :
Then, a plan, made from fixed landmarks like a wall, is attached to the photos to inform about the size of the places, their layout, access and their sense of openness. This plan can be used as a basis for the realization of a model or a virtual panorama with the software Stitcher used by the police. As for the gendarmes, they have a laser that digitizes the scene in 3D in which one can navigate at will.
10. Where to stop :
No question of brushing a whole apartment of magnetic powder or of Bluestar (solution which makes the traces of blood appear), but no question of missing a decisive index either. So where do we stop? "It is not a question of drowning the lab but of reporting the most pertinent traces possible," says Colonel Fombonne, in charge of technical and scientific training at the national gendarmerie.The number of samples also varies according to the type of " offense, usually about ten for petty delinquency and up to hundreds in the case of an attack ". Thus, if the technician's work stops at the door of the lab, he is aware of the stakes and takes into account the work and the cost of the analyzes while he makes the samples.