How Forensics Experts Protect the Chain of Custody
Every person who handles evidence must be accounted for and recorded as a link in an unbroken chain of custody, from crime scene to courtroom. Without a continuous record showing that evidence has been kept safe and secure from the crime scene to the lab and ultimately the courtroom, evidence may be rendered inadmissible in court.
Any competent defense attorney would rightly question the authenticity and integrity of any evidence for which outside contamination cannot be ruled out.
Whoever finds an item of evidence marks it for identification, which sometimes consists of writing or scratching his or her initials onto the item itself. Of course, this method is appropriate only when it won’t damage the evidence or alter any of its specific identifying characteristics. For example, an investigator may scratch his or her initials on the side of a shell casing found at the scene. In court, the investigator can positively identify the shell casing as the exact one uncovered at the scene.
Not all evidence can be marked directly, however. An investigator probably wouldn’t mark a bullet by scratching initials into it, because doing so can alter the striations on the side of the bullet that are used to identify the gun from which it was fired. Altering the bullet makes matching it with the gun more difficult.
Therefore, evidence like a bullet is placed into an evidence bag, which is marked and initialed by the person who finds it. The identifying information on the evidence bag includes the case number, the name and description of the item, the name and initials of the person who found it, the names of any witnesses to the discovery and recovery, and the date, time, and location of the find.
Some items require special packaging before being placed in an evidence bag. For example, a blood sample may be taken using a moist, cotton-tipped swab. After drying, the swab is placed into a sealed glass tube, and the tube is marked with the collector’s initials and date. The tube is placed into an evidence bag, which is similarly marked. The collector can then reasonably testify that that is the sample he obtained by identifying his initials on the sample tube and the evidence bag.
Each person who accepts an item of evidence initials or signs and dates the evidence bag and is then responsible for maintaining its integrity until it is passed along to the next person (or link in the chain). Here’s how it works:
A police officer finds a shell casing at the scene of a homicide; he collects it, marks it, places it into a marked evidence bag, and then signs it over to one of the crime-scene investigators.
The investigator transports the evidence to the lab and signs it over to the crime lab technician.
After the item is tested and evaluated, the lab technician signs the evidence over to the police department’s custodian of evidence (the officer in charge of the secured evidence lock-up area at the police department).
The custodian of evidence places the evidence in a secured area until it’s needed again. From there, it may be signed over to the prosecuting attorney for presentation in court.
If the chain of custody remains intact, each witness, from the officer who found it to the custodian of evidence, can testify that the item presented in the courtroom is indeed the item that was collected at the scene and tested by the lab.