sounds like a letter for the IRS. Maybe both groups work together.
The story is short on information and long on implication.
Mailed to every resident or every household?
Nobody noticed anyone hauling sacks of mail into the post office? Stuffing the mailbox on the corner?
On the other hand, what a great plot for an episode of NCIS, or CSI New York.
Invest in a couple of hundred bucks on threatening mail, then whack your victim and hope the sandmaggots get the blame.
Kind of a cute story.