Collecting Evidence

Blood left at a crime scene

After the CSI has created a full record of the crime scene exactly as it was when she arrived, it's time to collect the evidence. Now it's time to touch stuff -- very, very carefully. The CSI systematically makes her way through the scene collecting all potential evidence, tagging it, logging it and packaging it so it remains intact on its way to the lab. Depending on the task breakdown of the CSI unit she works for and her areas of expertise, she may or may not analyze the evidence in the lab.

Every CSI unit handles the division between field work and lab work differently. What goes on at the crime scene is called crime scene investigation (or crime scene analysis), and what goes on in the laboratory is called forensic science. Not all CSIs are forensic scientists. Some CSIs only work in the field -- they collect the evidence and then pass it to the forensics lab. In this case, the CSI still needs to understand the basics of forensic science. This way she can recognize the specific value of various types of evidence in the field.

Specialty Departments within a Crime Lab Crime labs can include several special departments. Often, a piece of evidence passes through more than one department for analysis. Here are a few examples of specialty departments: Latent fingerprints and impressions -- Develop latent fingerprints; analyze and compare fingerprints, footwear and tire impressions; run fingerprints through the Automated Fingerprint Identification System (AFIS, which utilizes the FBI database) for comparison against hundreds of millions of prints Chemistry -- Conduct analysis and comparison of illicit unknown chemicals and other materials Computer Crimes -- Recover evidence from computers; perform computer enhancement on audio or video evidence Firearms and toolmark identification -- Identify firearms; test firearms to establish barrel pattern and distance of gun from entrance wound; identify and compare bullets, casings and toolmark impressions Serology and DNA -- Conduct body fluid analysis, including DNA analysis for blood stains, semen and hair for identification and comparison

The crime lab processes all of the evidence the CSI collects at the crime scene. When the lab results are in, they go to the lead detective on the case.

As we mentioned earlier, the CSI has several main goals in mind when collecting evidence from a crime scene: Reconstruct the crime, identify the person who did it, preserve the evidence for analysis and collect it in a way that will make it stand up in court. Evidence can come in any form.

Body of Evidence Blood found at a crime scene can tell investigators a whole lot of things. Samples can be taken from clothing, furniture or other surfaces. For example, if there is dried blood on any furniture at the scene, the CSI will try to send the entire piece of furniture to the lab. A couch is not an uncommon piece of evidence to collect. If the blood is on something that can't reasonably go to the lab, like a wall or a bathtub, the CSI can collect a sample of the blood by scraping it into a sterile container using a scalpel. The CSI may also use luminol and a portable UV light to reveal blood that has been washed off a surface. The lab will perform DNA analysis of any submitted sample so it can be used later to compare to blood taken from a suspect. If there’s blood at the scene, there may also be blood spatter patterns. Spatter patterns can happen during a scuffle or fight. These patterns can reveal the type of weapon that was used during the scuffle. Analyzing a blood pattern involves studying the size and shape of the stain, the shape and size of the blood droplets and the concentration of the droplets within the pattern. The CSI takes pictures of the pattern and may call in a blood-spatter specialist to analyze it.

With theories of the crime in mind, CSIs begin the systematic search for incriminating evidence, taking meticulous notes along the way.

The actual collection of physical evidence is a slow process. Each time the CSI collects an item, she must immediately preserve it, tag it and log it for the crime scene record. Different types of evidence may be collected either at the scene or in the lab depending on conditions and resources, such as:

A footprint

• Trace evidence (paint residue, broken glass and unknown chemicals) - To collect trace evidence, a CSI might use tweezers, plastic containers with lids, a filtered vacuum device, a knife and a biohazard kit (which contains disposable latex gloves, booties, facemask and gown and a biohazard waste bag). Trace evidence is important.

• Impressions (fingerprints, footwear, tool marks) - Tools for recovering fingerprints include brushes, powders, tape, chemicals, lift cards, a magnifying glass and Super Glue. For three-dimensional impressions, like a footprint, a CSI will use a casting kit. A casting kit might include multiple casting compounds (dental gypsum, Silicone rubber), snow wax (for making a cast in snow), a bowl, a spatula and cardboard boxes to hold the casts. For toolmark impressions, a cast is much harder to use for comparison than it is with footwear. If it's not feasible to transport the entire item containing the tool mark, a CSI can make a silicone-rubber cast and hope for the best.

  Questioned documents

• Body fluids - To identify and collect these pieces of evidence, a CSI might use smear slides, a scalpel, tweezers, scissors, sterile cloth squares, a UV light, protective eyewear and luminol. He'll also use a blood collection kit to get samples from any suspects or from a living victim to use for comparison.

• Hair and fibers - A CSI may use combs, tweezers, containers and a filtered vacuum device to collect any hair or fibers at the scene.

• Questioned documents (diaries, notes, phone books; also includes electronic documents like answering machines, cell phones, PDAs and caller ID units) A documents lab can often reconstruct a destroyed document, even one that has been burned, as well as determine if a document has been altered. Technicians analyze documents for forgery, determine handwriting matches to the victim and suspects, and identify what type of machine was used to produce the document.