The simplest and most likely answer is that they didn’t want to know. If they can say “he had bumpstocks” they had reason to ignore the fact that bumpstocks are 100% legal and ban them anyway without legislation. If they had found out that he genuinely modified them to be real machine guns, which are already banned by legislation, then they wouldn’t have their justification for going outside the law. There might be another answer but this is the one that feels the least like a conspiracy theory. It took a FOIA request for them to even admit that they were prohibited from inspecting any of the weapons used.
The simplest and most likely answer is that they didn’t want to know. If they can say “he had bumpstocks” they had reason to ignore the fact that bumpstocks are 100% legal and ban them anyway without legislation. If they had found out that he genuinely modified them to be real machine guns, which are already banned by legislation, then they wouldn’t have their justification for going outside the law. There might be another answer but this is the one that feels the least like a conspiracy theory. It took a FOIA request for them to even admit that they were prohibited from inspecting any of the weapons used.