Plot hole: Clint Eastwood's whole investigation/solution is based on him suspecting that the victims had a common tie, rather than being random. The problem is there is no reason for him to think this - there is no reason given for them to not be considered random victims. For him to think otherwise was a bit far fetched. In the book, however, they were all on the organ donor list, which led him to the fact they all had the same rare blood type and so on.
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Suggested correction: Not really farfetched. It's a common plot device for a cop to look for a link. When he finds out Gloria gave blood, he remembered the sticker in the other victim's car.
