Chain of custody: who held it, and when?

Imagine a lab finds a perfect match on a shoe print cast. Then someone asks: where has the cast been for the last three weeks? If the answer is "nobody wrote it down", the match is worth very little. The question "did this item change?" must always have a documented answer.

The chain of custody is a written record of every person who held an item, with the time, the place and the reason. It begins at the scene and ends when the item is no longer needed.

How it works.

• Each item goes into its own package (items are never mixed), labelled with the case number, a description, the collector's initials and the date and time.
• The package is closed with a tamper-evident seal, and the collector signs across the seal. A broken seal shows that someone opened it.
• Each time the item changes hands, both people sign and time the log, and the log says why.
• Damp items are dried before packing, because moisture can spoil them, and dry items usually go in paper or card, not plastic.

A sample log for one item, a strip of tape from the staff board:

• 09:05, collected in Room 3 by Officer A, sealed in bag 1127, initials across the seal.
• 09:50, handed to the evidence clerk, Clerk B, seal checked and intact.
• 14:10, signed out to Examiner C for fingerprint development, seal opened.
• 14:45, returned and resealed by Examiner C in a new bag, 1131, the old bag attached to the log.

Anyone reading this log can see every hand the tape passed through and every time it was opened.

What if the chain breaks? A gap does not make the item false. It makes it less reliable, because nobody can show it was not swapped, contaminated or damaged. Depending on the rules of the place, an item with a broken chain may be refused as evidence, or may carry less weight.

Contamination is the other danger. A bare hand adds your own prints and DNA. A dirty tool spreads traces from one item to the next. That is why gloves are changed between items and why elimination samples are taken from people who had a right to be at the scene (staff, first responders), so their traces can be recognised and set aside.