Crime scene
Chain of custody in forensic science: steps, records and common failures
What the chain of custody is, the steps that keep it unbroken from crime scene to courtroom, what a custody record must show, and the mistakes that get evidence thrown out.
Forensic Science Pakistan2 min read
A laboratory result is only as strong as the proof that the exhibit tested is the one recovered from the scene. The chain of custody is that proof: an unbroken, documented record of who handled the evidence, when, and why.
What chain of custody means
NIST defines it as a process that tracks evidence through collection, safeguarding and analysis by documenting each person who handled it, the date and time it was collected or transferred, and the purpose of each transfer. If a link is missing, the defence can argue that the evidence was swapped, contaminated or tampered with.
Why it matters in court
Pakistani courts treat forensic reports as expert evidence that must be connected to the case, and the Supreme Court has required DNA samples in rape cases to be preserved. A strong DNA match can be undermined by a gap in custody, as our article on DNA evidence in Pakistani courts explains.
The steps, from scene to court
- Secure and document the scene: photograph and record each item where it lies before anything is moved.
- Collect: wear gloves and change them between items; use sterile swabs for biological traces.
- Package: put each item in its own container; use paper, not plastic, for damp biological evidence so it does not rot.
- Seal and label: seal with tamper-evident tape, and sign and date across the seal. The label records the case number, item number, description, place, date, time and collector.
- Record every transfer: each hand-over is signed by the person giving and the person receiving the item, with the date, time and reason.
- Transport and store: keep biological samples cool and dry, and store exhibits in a locked, access-controlled area.
- Laboratory intake: the laboratory checks the seals, records their condition and assigns its own reference.
- Analysis: analysts record when they open, sample and reseal the item.
- Court and disposal: the item and its record are produced in court, then retained or returned as ordered.
What a custody record must show
- A unique case and item number that appears on the exhibit and every document.
- A description of the item and where it was found.
- The name, signature and role of everyone who held it.
- The date and time of every transfer, and its purpose.
- The condition of the seal at every opening.
Common failures
- Several items packed together, which allows DNA to transfer between them.
- Biological evidence sealed in plastic, where mould destroys the DNA.
- Unsigned or undated transfers, or items left in an unlocked office.
- Seals opened without a record, or labels that do not match the exhibit.
- Samples sent to the laboratory weeks late, after they have degraded.
Chain of custody for DNA
Modern DNA tests can detect a few cells, so contamination is the main risk. Separate collection kits for each item, protective clothing and staff elimination samples help, and the custody record shows that those precautions were taken. See how a DNA case is processed for the laboratory side, and our guide to types of fingerprints for developing marks at the scene.
We train police and investigators in evidence collection and custody. Contact us to arrange a workshop.
