if the police get a report and actually look into it, i would imagine whoever is sending those could get arrested pretty quickly.
their fingerprints are probably all over those letters.
How exactly do people send letters in the US? A quick search gives me the impression that someone can drop a letter "anonymously" and the post office will send the letter to the recipient's address just fine. Using a glove or something would hide the fingerprint, too, although I'm not sure if a smart scammer would go this route instead of spamming spam emails to thousands of email addresses.