Criminal investigation and forensic science

Forensic scientist conducting a bloodstain pattern experiment

A crime is committed when there is a breach of the criminal law. There are two elements of a crime that must be present to convict an accused: the guilty mind (referred to as “mens rea”) and the guilty act (“actus reus”). Forensic science is the best means of establishing the physical element (i.e. the actus reus) of a crime. The mental element (mens rea) can only be inferred from the physical evidence and surrounding circumstances of the case. This article seeks to briefly explain how forensic science can be used to support the investigation and prosecution of a crime.

The investigation of a crime can be described as the process by which the police or investigating officers gather facts and information or evidence that can be used to address the following key questions:

  1. What crime has been committed?
  2. Who is (are) the perpetrator(s) of the crime? Who is (are) the victim(s)?
  3. When did the crime occur? What were the sequence of events?
  4. Where did the crime take place?
  5. How did the crime happen?
  6. Why was the crime committed? What was the motive of the crime?

Answering the questions above will lead to the development of further specific questions or lines of inquiry. For example, in an alleged rape case, addressing what crime has been committed will require the investigator to determine if there was sexual intercourse and whether there was consent. The investigators will gather facts and information by examining vaginal/anal swabs for semen, sperm and DNA (- to address penetration and identify the perpetrator), examining victim for injuries and clothes for damage and tears (- to address consent), recovering drugs and carrying out other scientific analysis such as toxicology and alcohol analysis, and interrogating suspects, victims and witnesses.

Forensic science can simply be described as using scientific techniques to answer the investigative questions: what?, who?, when?, where?, how?, why? Over the past two decades, forensic science has proved to be the most effective means of addressing the main questions in a criminal investigation. Forensic science is more objective in answering the lines of inquiry and securing a conviction than traditional investigation methods such as interrogation of suspects and witnesses. Through the establishment of forensic intelligence databases, investigators are able to solve crimes faster, identify unknown offenders and link crime scenes.

In Ghana, the police rely mainly on traditional methods of investigation such as gathering information from people and the public to answer lines of inquiry. The use of information from physical/forensic evidence is highly limited. This is perhaps partly due to the limited trained forensic experts in the criminal justice system, poor scientific support in physical evidence identification, collection, preservation and analysis, and lack of forensic intelligence databases such as National DNA and fingerprint databases. Another problem is that the police or investigators, and other members of the legal profession (e.g. lawyers and judges) may not fully understand the investigative potential of forensic evidence and its relevance in the prosecution of a crime.

Forensic evidence can be used to answer the most important questions in different types of crime. DNA and fingerprints can be used to identify (i.e. addressing who?) and link individuals (suspects, victims and witnesses) involved in a crime. Both DNA and fingerprints can be considered “unique” to an individual and can conclusively identify or eliminate suspects. DNA evidence can be particularly important in cases where there are no witnesses, unknown suspects, unidentified bodies or missing persons. Impression evidence (e.g. tool marks, shoe marks, tyre tracks, bite marks and marks on fired bullets) can be used to identify objects or items used in a criminal activity.  It can establish direct contact between two objects and indirectly link individuals who possess the objects or items to the crime scene.

In cases of drug trafficking or possession of drugs, and drug-facilitated sexual assaults, forensic chemists can identify the substances involved by carrying out drug analysis and toxicological analysis. Forensic chemists can also examine the pattern of damage during fire-related incidents to determine the point(s) of origin of the fire. The cause of the fire can be determined by identifying substances such as flammable liquid, explosive materials and faulty electrical appliances. Information obtained from fire investigation can help the police to conclude whether the fire was accidental, natural or deliberate.

Analysis of trace evidence such as hairs, fibre, glass, soils, paint flakes, pollen, wood chips, plant debris, feathers and dust can be used to link individuals, places, and items involved in a crime.  Trace evidence are characteristically small in size and are easily transferred from one location to another. It is very unlikely for a perpetrator to know that such traces have been left at the scene. In crimes against the person (such as homicide, rape or violent assaults), victims can be associated with suspects using hairs and fibres, as well as blood and other body fluids and DNA.

To determine where and when a particular crime was committed, digital forensic scientists can conduct cell site analysis to locate the vicinity or trace movement of a mobile phone. This can be done by analysing data from calls made or received or text messages. This technique can be very useful in robbery cases where mobile phone communication is very common. Cell site analysis can corroborate or refute statements of witnesses and suspects. Shoe marks, bloodstains, trace evidence such as fibres, pollen and plants can also be used to determine where a crime was committed. Bloodstain pattern analysis can be used to reconstruct the sequence of events in an alleged crime. It can also provide information about the nature and location of attacks, the type of weapon used and the degree of force applied.

Overall, forensic science is an important tool for investigation and prosecution of crime. It can help identify individuals and substances, link people, places and things, corroborate statements or prove an alibi. The surge in crime activities in Ghana requires a scientific crime fighting approach. Law enforcement agencies must ensure that forensic experts are integrated into the investigation team and scientific support is available to investigating officers throughout the investigation process. The government and the major stakeholders must invest in building adequate resources for forensic practice in Ghana.

By: Aaron O. Amankwaa (BSc (Hons), MSc), Editor & Contributor (Forensic Science & Biochemistry)

©2016 Scientect e-mag | Volume 1 (1): A6

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