Evidence Gathering by Police

Evidence gathering is the process by which police collect, record, and preserve material that may be relevant to a criminal investigation. The integrity of the evidence-gathering process is critical: errors in collection or handling can result in evidence being excluded at trial, or in wrongful convictions or acquittals. The principles governing evidence gathering in England and Wales are found in statute (principally PACE and the Criminal Procedure and Investigations Act 1996), in the PACE Codes of Practice, and in guidelines published by the College of Policing and the Attorney General.

Physical evidence such as fingerprints, DNA samples, fibres, and trace materials must be collected in accordance with forensic science protocols. Chain of custody must be maintained from the moment evidence is seized to its presentation in court. Each item is labelled, bagged, and logged, and any transfer of custody is recorded. Contamination of forensic evidence can render it inadmissible or unreliable, and officers are trained to avoid touching surfaces, wearing appropriate protective equipment, and securing scenes as quickly as possible.

Digital evidence has become increasingly significant in criminal investigations. Phones, computers, tablets, and other devices may hold messages, call records, location data, images, and social media activity relevant to an offence. The seizure and analysis of digital devices must be conducted in accordance with the ACPO Good Practice Guide for Digital Evidence (now the Digital Forensics sub-committee of the National Police Chiefs' Council). Hash verification is used to demonstrate that a device's contents have not been altered during examination. Material extracted from devices must be disclosed to the defence in accordance with the Criminal Procedure and Investigations Act 1996.

Witness evidence is gathered through the PEACE interview model for witnesses, and written statements taken under section 9 of the Criminal Justice Act 1967 form the basis of the prosecution evidence file. Body-worn video cameras have transformed witness evidence gathering: footage captures events as they occur, provides a contemporaneous record, and reduces disputes about what was said or done at a scene. Officers are required to activate their cameras at the earliest opportunity and must not edit or delete footage. Unused material, including footage that does not support the prosecution case, must be disclosed to the defence.