Criminal Investigations are the First Part of a Criminal Defense Case
The primary duty of law enforcement officials is to protect and serve the citizens of America. Police officers and detectives are tasked with investigating crimes, and their primary objective is to make an arrest in the case. The techniques used to catch criminals have become much more sophisticated over the years. With the advances made in technology, law enforcement now has many weapons in its arsenal that are proving to be instrumental in solving cases. It is important to understand how investigators and prosecutors gather evidence in building a criminal case against the defendant. Most people get caught because they don't realize how minute traces of evidence left behind can be analyzed and used to convict them in a criminal case.
How Crimes Are Investigated
How a crime is investigated largely depends upon what type of crime was committed. First responders are responsible to keep unauthorized people away from the scene. They should seek out people who can provide details about what happened. These witnesses are often the cornerstone in a criminal case. Homicide detectives are responsible for securing the crime scene in an effort to protect physical evidence from contamination when a murder has occurred.
Investigating officers will begin by gathering a variety of evidence and placing it in bags. Crime scene units are called to take photographs, dust for fingerprints and perform blood spatter analysis. They make an effort to collect hairs, fibers, dried blood and bodily fluids that may prove relevant to the case. Crime scene investigators will often use a vacuum to collect minute pieces of trace evidence. All of the physical evidence is then taken to a forensic lab for testing.
Crime Scene Investigators in Real Life
Most Americans are now familiar with forensic testing and the procedures used to solve crimes. Television shows such as "Crime Scene Investigation" have become more popular in recent years. Crime scene investigators conduct an initial walkthrough to get an overall feel for what happened. They will visually examine the scene and come up with theories of how the crime occurred. To aid in their investigation, a series of techniques may include the following:
A bullet trajectory kit may be needed to determine the source and trajectory of the bullets fired. A unique laser-pointing device is attached to a protrusion rod and the laser beam indicates the direction and angle of the source. In addition, shell casings left behind can help determine the type of weapon used and who handled the ammunition if fingerprints were found on the casings.
Similar to basic fingerprinting, this technique is used for obtaining impressions from wood or tile floors. Powder is applied using a brush to make the print more visible. A lifting machine or tape is then used to record a visual of the impression.
The primary purpose of casting is to recover large three-dimensional impressions such as footprints or tire marks left at the scene. The way the process works is similar to how an orthodontist makes an impression of a patient's teeth. A substance is poured into the area, allowed to harden and then removed.
These are miniscule amounts of material found at a crime scene that are almost invisible. No matter how much effort is made in cleaning up, there is always something left behind for investigators to find. Trace evidence may not be enough to make a case, but it acts to corroborate additional evidence in the case.
Ultraviolet (UV) Light
A handheld UV device is used to find evidence of bodily fluids such as blood or semen. These are often undetectable to the naked eye but show up easily under a UV light.
This unique compound is used to reveal hidden crime scenes. The chemicals in luminol mix with the hemoglobin in blood, causing it to fluoresce. After luminol is sprayed, traces of blood will emit a blue glow. Blood spatter patterns help investigators to locate the point of attack and identify what sort of weapon was used in the crime.
Deoxyribonucleic acid (DNA) contains genetic information about a person. It is contained in blood, semen, hair, mucus, skin cells, fingernails, teeth, saliva and organs. DNA is organized into long thin structures known as chromosomes. Every cell in a person's body has the same DNA, which is primarily stored in the cell nucleus. A strand of DNA is made up of four different building blocks:
These pair up with each other to form units that are called base pairs. Each person's DNA is different from everyone else except for identical twins. By collecting DNA evidence, a suspect can either be linked to a crime or eliminated altogether.
How DNA Is Used to Solve Crimes
There are a variety of forensic scientific techniques used to gather and examine evidence. Crimes are solved by using a series of DNA tests. There are several different types of DNA testing that forensic scientists perform that include:
Restriction Fragment Length Polymorphism (RFLP)
This technique is used to prepare a DNA sample for analysis. It is exposed to restriction enzymes that recognize and cut the DNA sample at specific sequences. These fragments are different lengths and half will match the mother's DNA, while the remaining fragments will match the father's DNA. RFLP has a higher rate of exclusion, but the downside is that a large blood sample is needed and it takes much longer to obtain the results.
Polymerase Chain Reaction (PCR)
This method is used to analyze a short sequence of a DNA sample that may only contain minute quantities. PCR is used to reproduce specific sections of DNA and it takes only a few hours to get a result. This is a highly valuable technique because unlimited copies of DNA can be reproduced. The method uses thermal cycling by repeatedly heating and cooling test tubes that contain the reaction mixture. This allows a single piece of DNA to be amplified very quickly.
Short Tandem Repeat (STR)
This type of analysis is used to evaluate specific regions within the DNA. The variability in the regions is used to distinguish one DNA profile from another. The Federal Bureau of Investigation (FBI) uses a standard set of 13 STR regions that are used by the Combined DNA Index System (CODIS). This large database contains DNA profiles that are used to match evidence found at a crime scene.
Mitochondrial DNA (mtDNA)
All mothers have the exact same mitochondrial DNA as their children, because the mitochondria of each embryo comes from the egg of the mother. The father's sperm only contains nuclear DNA. The mtDNA profile is compared to determine if the person shares certain regions of DNA that indicate they came from the same maternal line.
The Y chromosome is passed from the father to his son. By analyzing these genetic markers, the Y chromosome is used to trace male relationships.
DNA is very powerful evidence because matches are made based on a sequence of repeating numbers. Murders can be solved by simply matching saliva or teeth impressions to a bite mark on the victim. This is the type of evidence that was used to convict serial killer and rapist Ted Bundy in 1979.