There has to be more than 1 shifty person in the city at any given point right? Maybe they did see someone acting weird down the street- but that person was robbing an apartment and isn't the murderer.Tracking and clearing suspects should be a more integral part of the overall gameplay.
Also- if the witness knows the suspicious person's name they should be able to tell you it, or have the option to withhold information (good friend/spouse/boss/whatever situation.)
If they got a good look at them (ex- was near the person for an extended period of time)- could tell you more than 1 feature, ideally something like pulling 1 or 2 out of a "height, hair color, rough age, action, etc" pool of potential traits of their suspicious person, allowing for multiple witnesses to tell you differing facts about the same person- or about 2 separate suspects, you will have to check when and where the suspicious person was seen.
If we end up having the ability to make a "blank suspect profile" in the future, you could start filling that out, and ask other potential witnesses if they've seen a "heavyset bald dude" and if they have, see if they may happen to know them.
Sort of like the current "Unknown Citizen" evidence that pops up for side jobs.