How Crime Scene Investigation Works

Analyzing the Evidence: Forensic Science
On the Stand
The role of a crime scene investigator doesn't end when he completes his evidence report. It doesn't even end when the lab results related to that evidence are delivered to the detectives on the case. A big part of a CSI's job is testifying in court about the evidence he collected, the methods he used to recover it and the number of people who came into contact with it before it ended up as the prosecution's Exhibit D. And the defense attorney's job is to attack the evidence, which sometimes means attacking the person who collected it. This is why search warrants, evidence logs, photographs and extremely detailed reports are so critical to the CSI process. The defense will try to get every piece of incriminating evidence thrown out of court. The legality of the search, the untainted preservation of the evidence and the full, undisputable documentation of the crime scene are prime considerations in a crime scene investigation.

­The first forensics lab in the United States opene­d in 1923 in Los Angeles. In 1932, the FBI established its own forensics lab to serve police departments and other investigating authorities all over the country. The FBI lab is one of the largest in the world.

The Denver Crime Lab at the Colorado Bureau of Investigation provides evidence collection and laboratory analysis for any police department in Colorado that requests its services. It also conducts state investigations that don't fall under the jurisdiction of any local authority.

Some specialty departments in the Denver Crime Lab include:

  • 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

    CBI technicians use these camera setups to photograph recovered prints to use in comparison and for running through the AFIS system. On the left is an old-school Polaroid setup, and on the right is a digital-camera setup. Mr. Clayton prefers the Polaroid results.

  • Trace evidence
    Run GSR analysis; identify and compare samples of soil, glass, fibers and paint
  • Chemistry
    Conduct analysis and comparison of illicit drugs, explosives and unknown chemicals
  • 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

    Comparison microscope setup in the CBI serology lab

  • Questioned documents
    Detect forgery and alterations; conduct handwriting comparisons; reconstruct destroyed documents; identify and compares printers, typewriters or copiers used to produce a document

Often, a piece of evidence passes through more than one department for analysis. Each department delivers a complete report of the evidence it analyzed for the case, including the actual results (numbers, measurements, chemical contents) and any expert conclusions the scientists have drawn from these results. The CSI in charge might compile the results and deliver them to the lead detective on the case, or the lab might send the results directly to the detective squad.

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