When a crime is committed forensic experts are summoned to discover exactly what has happened. Their success at gathering evidence depends on many factors, from the reliability of signs present to examine, to the sophistication of equipment available. How forensic experts collect evidence is dependant their level of expertise, and time passed since a crime scene was found, along with how much freedom and access they have to establish facts.
Forensic experts begin examining a crime scene by taking photographs. Photographic evidence can be used to support theories later on in a case, and help show exact details, rather than experts having to rely on memory. So that they do not contaminate the scene, experts will usually wear gloves, and possibly disposable booties.
Next, large items present at the scene, such as clothes, are placed separately in paper bags and accurately recorded. Several forensic experts may work together and each performs a separate task. Usually, one expert gathers items that belong to a victim, while a different expert collects items that belong to a suspect. This makes confusion and contamination less likely to arise.
While items are being gathered, forensic experts are careful not to lose or damage evidence. Evidence such as hairs, skin, and bodily fluids may hold the key to a crime and need to be handled professionally in order for them to hold value.
Evidence from a floor surface, or the surface of an object such as a chair, may be gathered by taping or by vacuuming the area so that contents can be examined later. Debris captured via vacuuming is emptied out onto a sheet of white paper, the corners of which are folded over, and then sealed in a bag.
Standards, such as pet hair or carpet fibers which may be transferred to a victim, are gathered separately in sealed bags or envelopes.
Fingerprints may be searched for at a crime scene by a process of dusting areas to reveal those left by a victim or suspect.
Vehicles that may have been used in a crime are dusted for prints and evidence carefully obtained. If a crime scene involves evidence such as tire tracks, forensic experts may take a cast of the impressions left.
Fingernail scrapings from a victim are taken and hair samples from their head and pubic region.
Entry and exit points at a crime scene, such as windows and doors, are dusted for prints and checked for traces of evidence.
Forensic experts have to pay great attention to detail when gathering evidence of a possible or definite crime. Negligence and a lack of care could lead to a case being dismissed or inaccurate test results obtained when evidence is taken back to the laboratory.
Sources
http://www2.fbi.gov/hq/lab/fsc/backissu/july2000/deedric4.htm#Evidence Collection
