Full autos are legal with a class III license and a $200 tax stamp for each weapon. Class III license requires FBI in depth background check.
Presuming he owned them legally, which we don’t know yet.
Several years ago, while working part time at a gun store, a “customer” came in and offered to sell me an M-60, complete with tool kit and extra barrel. Ten grand, no questions. I hastily declined thinking it may be an ATFE “trap”. But the possibilities....they are out there.