What happens to the material
Biological traces break down from heat and moisture. A bag left for years in an uncontrolled basement may become unsuitable for analysis even if it looks intact. Storage rules were introduced long after the storage facilities themselves.
The second reason is moving and disposing of materials. Archives are consolidated, case files are transferred, and some materials are destroyed when the established retention period expires. When a new method of analysis appears 20 years later, there may be nothing left to apply it to.
The third reason is the work done on the material itself. Each test uses up part of a sample, and older methods required more. Sometimes all that remains of the evidence is an expert report from the last century.
What has changed
The arrival of more sensitive methods forced the rules to be rewritten. Evidence is now stored longer and under controlled conditions, and handling procedures have become stricter: every access is logged, and opening the packaging is documented.
- Store materials under controlled conditions, not in basements
- Log every time evidence is handled: who, when, why
- Leave part of the sample untouched for future methods
- Automatically destroy materials from serious cases when the retention period expires
- Work with an old sample without accounting for possible contamination
- Treat a report from the last century as equivalent to the sample itself
The third item on the left became standard precisely because new methods kept emerging. Experts now deliberately avoid using up all the material: what cannot be analyzed today may become decisive 20 years from now.
The second item on the right explains some failures during case reviews. Samples opened repeatedly in workspaces without protective measures can contain traces from other people, and it may no longer be possible to determine whose they are.
There is also the opposite challenge: volume. Storing all the material from every case for decades requires space and money, so decisions about what to keep have to be made in advance, without knowing which case will be reviewed.
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Sources
- The suitability of physical evidence for repeat testing depends on storage conditions and retention periods
- modern rules provide for controlled storage, documented access, and preserving part of the sample for future analytical methods