Eyewitnesses have notoriously bad memories. Many studies have been conducted where a group of people watch a simulated crime and are then asked to describe the details of the event. It is very rare for the criminal to be described accurately. Perhaps this is because people always have a difficult time describing a face, even if it is right in front of them. Faces do not have features that can be easily articulated with vocabulary that we posses. How would you describe your own nose to someone over the phone?
Police use artists and kits full of different face parts to help witnesses piece together a face. Even with this help, an accurate portrayal of the suspect is rarely achieved. In fact, two eyewitnesses to the same crime often produce wildly different portraits. Perhaps this is because memory can get distorted under stress, or maybe it is because when people are trying hard to remember something, they will sometimes fabricate a false memory to satisfy themselves and their questioner.