How the hell can know what weapons he purchased, where, what time etc?
I thought those records were protected? /S
Any firearm purchased from a dealer will have that info stored for a minimum 20 years (most probably keep them longer). Dealers are required to keep a log of sales that has all 4473s in it. If they go out of business, that log is sent to the ATF for storage. So all these guns traced to the guy are probably the ones they found in his room, and by talking to the manufacturer, they find the store, who then has their logbook where the sale is recorded. Any LEO probably can trace any serial number to it’s original purchaser, and from there has to go talk to people and find where it went from there. Sell through a ffl, personal sale, still owns, etc.