In watching the local TV report video, I notice the threat letter is computer printed. If the Postal inspector really wants to solve who sent the letter, which I’m dubious, it will likely be pretty simple.
Virtually *every* printer made prints a Machine Identification Code on every document printed. The code consists of nearly invisible yellow dots. Find the dots and you find the serial number of the printer, which can then be used to trace who purchased the printer.
https://www.instructables.com/Yellow-Dots-of-Mystery-Is-Your-Printer-Spying-on-/
Tell us how the millions of black toner laser printers in use are going to print yellow dots?