It was an inside job, per the article and except above:
A longtime guard at the ATF facility has admitted to carting off thousands of firearms, gun parts and ammunition and selling them over several years.
So we have 1) a deliberate criminal act; and 2) a leadership/management and procedural failure of the first order. This guy should have been caught through inventory and auditing control measures years ago.
inventory and auditing control measures
I asked him if I could have them, and he said, “Hell, no, man. We got to *inventory* that stuff!”