<i>&quot;Or he&#x2F;she was a pro and left the casings and a chunk of the gun on purpose to cast doubt on the idea that he&#x2F;she was a pro.&quot;</i><p>I think a more likely explanation would be that the killer was a pro, and didn&#x27;t care about the brass since they planned on dumping the weapon, which couldn&#x27;t be traced to them anyway.