“American citizens cannot own brand-new full-auto rifles, so I doubt they’re of American manufacture.”
Americans can and do own and manufacture automatic weapons, silencers, cannons, ect. Sometimes you need a license, sometimes not.
Not automatic weapons made since 1986. I sort of doubt these were that old. Or that expensive either. I'll bet they did not say "Colt" or "FNC" either. Maybe Chinese clones. But the question remains, were they headed to Mexico, or from Mexico to the Drug gangs US based branches?
After 1986, Americans don’t legally own new full autos.