They have been mailed from the Streator area to individuals, businesses and government offices.
Mesarchik was abducted from his front yard in Streator just over seven years ago.
The child's severely beaten body was found in the Vermilion River the next day. The boy's mother says -- at the least -- she hopes news of the letters will inspire other parents.
"Even if it doesn't lead to anything in Dalton's case," she said, "my hope is that it opens up a dialogue in families that they will sit down and talk to their children about being safe."
Investigators have not released information about the content of the letters. But they want anyone who has received one to contact them immediately.