All I know is what I have heard from the cops and the media. And the parameters are simple - the cops had reason to enter the tag into the database multiple times, for whatever reason. Do a query, and run down the top 200-500 plates by total entries. The investigation then goes from a needle in a haystack to a manageable list - that is exactly how they got started in catching the Center City (Phila.)/Fort Collins rapist - they entered in just about anyone who was in both Philly and subsequently Fort Collins and vicinity during the two timeframes (I could possibly have been considered, as I fit that intital geographic parameter), and then winnowed down that list to about 200 suspects - and the eventual perp was on that list. That is why they create a database in the first place, but apparently the FBI was too busy looking up ten-year-old gun purchase records and harassing law-abiding citizens to bother using the tools at hand.