Physical evidence

The three physical evidence strategies are built on the foundation of Locard’s Principle of Exchange, which states that ‘every contact leaves a trace’. They are:

• crime scene strategy (see Scene strategy)
• forensic strategy
• pathology strategy

These three strategies are intricately interwoven and investigating officers should keep the objectives and outcomes of all three areas in mind when developing, reviewing and managing these strategies.

Types of physical evidence

• Physical material, for example, what has the offender touched and how was entry gained?
• Victim incapacitation, for example, what weapons were used?
• Sexual evidence
• Forensic awareness, for example, did the offender wear gloves?
• Offender injuries, for example, was the offender injured?
• Missing items, for example, what was stolen?
• Blood distribution, for example, which escape route was taken?
• Linking evidence, for example, what evidence would link the offender to the scene?

Forensic strategy

The forensic strategy enables the potential of material recovered during crime scene examination to be maximised, and can help with:

• clarification of the circumstances – providing movements of victims and suspects, establishing crime scenes and attack sites, and challenging assumptions
• elimination of suspects – through partial DNA or partial palm prints
• forensic intelligence – providing potential links at scenes, DNA profiles from body fluids
• inceptive intelligence – the identification of an unknown person or fact, for example, identification through fingerprints or via DNA deposits
• context – once evidence has been obtained, establishing where it fits in the context of the whole investigation or examination
• clarification of the sequence of events – through, for example, analysis of blood distribution or the use of fire investigation units
• corroboration – including independent confirmation of circumstances, critical fact or witness testimony and evidence of the culpability of a suspect

Forensic samples may provide information for use during interview to test the reliability of an account. They may also assist in prioritising lines of enquiry or submitting particular items for examination.

The investigating officer should also consider both examination of results and exhibits management.

Assistance can be obtained from:

• crime scene investigators (CSIs)
• crime scene managers (CSMs)
• force forensic advisers
• forensic search advisory group via the NCA Major Crime Investigative Support

For serious or complex investigations, more information on setting a forensic strategy can be found in managing major investigations (forthcoming).

Examining results

The investigating officer should carefully examine forensic results to determine their meaning. To confirm or help with interpretation, the investigating officer should seek advice and clarification from a CSM, forensic liaison or the forensic provider.

Exhibits

An exhibits officer is required in serious or complex investigations, as the number of items or exhibits may be substantial.

For further information see Exhibits management.