I’m sure CB was on the list of 600 suspects because of his extradition. From there, an analysis of his prior criminal record would have been done. Then, was there anything to connect him to the area of the crime at the time it took place?
There are probably other criteria but even on this, CB perhaps wouldn’t stand out as a strong suspect, IMO, especially when we don’t know what the other 599 looked like. Even RM for example, who has alleged sexually deviant behaviour as a young man, is a stronger suspect based purely on his location at the time of the crime.
I agree that if CB is the culprit, it’s a terrible oversight but LE has to funnel information in an investigation and deal with the strongest suspects first and I think we can be certain, given CB’s record at the time, a large portion of the list were stronger suspects than him. Just MO.