Final answer:
Shell casings can help identify the perpetrator because they carry unique firearm markings and may contain DNA that can be matched with databases.
Step-by-step explanation:
Shell casings found at a crime scene can indeed assist in identifying the perpetrator. This is because the firing pin of a gun leaves unique markings on the primer of the casing, which can act as a fingerprint for that specific weapon. If the gun has been previously used in a crime or registered, the police can match these markings to their database to help identify the gun owner. Additionally, DNA may be retrieved from the casings if the perpetrator handled them without gloves. The DNA can be sequenced, analyzed, and then match it with a database, potentially finding a match to a known individual, helping lead to the perpetrator.