Physical Evidence and Collection Methods

Physical evidence is anything tangible that can link a suspect to a crime scene. This includes bodily fluids, DNA, fingerprints, and various materials left behind during criminal activity.

The foundation of physical evidence collection rests on Locard's Exchange Principle, developed by Edward Locard in the 19th century. This principle states that "every contact leaves a trace" - meaning criminals always leave something behind or take something with them from a crime scene. Paul Kirk reinforced this idea, noting that whatever someone touches or steps on becomes a "silent witness" against them.

Scene of Crime Officers (SOCOs) follow strict protocols when collecting evidence. Blood must air-dry at room temperature before being stored individually in evidence bags to prevent contamination. Similarly, semen traces are dried and double-bagged in paper and polythene bags. All evidence must reach forensic labs within 24 hours.

Key Point: Proper evidence handling prevents contamination and ensures reliability in court proceedings.

Hair samples, collected with tweezers, can reveal ethnicity and body location through DNA analysis. Bite marks represent "impression evidence" and are photographed or cast to match against dental records, whilst saliva provides additional DNA evidence through swabbing techniques.

Fingerprints and Impression Evidence

Fingerprints come in three distinct types, each requiring different collection methods. Patent prints (also called positive prints) are visible impressions left in blood or debris - these are simply photographed and analysed.

Latent prints are invisible impressions made by skin oils. These require special powder containing magnesium and adhesive chemicals to become visible, then they're photographed and lifted using sticky sheets preserved on acetate. Plastic prints are three-dimensional impressions left in materials like clay or soil, which can be photographed and cast for analysis.

Shoe prints provide valuable information about a suspect's gender, approximate height, and shoe size or brand. Impressions in soil can be moulded, whilst those in liquid surfaces are photographed and stored in the National Footwear Reference Collection for future reference.

Fibres represent another crucial form of trace evidence. Using tweezers, investigators carefully collect delicate threads from floors, clothing, and furniture. Since fibres are unique due to their synthetic or organic colours and manufacturing processes, they offer particularly valuable evidence.

Case Study Alert: The Jill Dando case revealed serious flaws in evidence collection when Barry George's conviction was overturned after forensic experts couldn't definitively link gunshot residue to him.