They don’t live in Minneapolis, they live in ST. ANTHONY, MN! The IRONY of THAT!!! Their town is named for a Christian saint!
If the letters were mailed, then there might be fingerprints on the envelope, the notes, and DNA on the stamps, envelope seal if licked, etc.
Might be worth getting it tested and if found, announce that the psycho’s DNA is now in the hands of the police and local residents who will try to match it to suspects in the community. The same for an attempt to match the handwriting with other letters/notes etc people in the neighborhood have from others there.
Worth the initial effort to scare the crap out of them.