Real or prop

Yesterday when I opened my mail box, I found a letter posted to my studio address but not any recipient. Being curious in its handwritten address, I opened it immediately. To my surprise, it's so much just like the imagined "props" of a triad movie which I exhibited in the 14QK show at Para/Site a couple of weeks ago. But this is not in a movie or an art exhibition. It's a piece of A4-sized paper, on which there are handwritten Chinese words "1819 burns you to death. Leung Chi Wo". Overleaf there's a no. which seems to be an HKID and the name of a person "Fung ??? On" whom I could hardly know. It's really mysterious, not far away from a movie plot indeed. A couple of burnt "joss notes"(usually as offering to the dead in Chinese ceremony) are enclosed. Could it be a joke or threat? But for what? For the first time, I reported a criminal case to the police for its possibility of a real threat, or to get a clue of it. As you could imagine like what you have seen in the movie, I was interviewed by a CID detective in a small room. They're serious about it and started investigation right-away like sending this letter for forensic test for finger prints.
Of course I hope it's only a joke but I don't think it's funny. Never thought this could happen in my real life! And what happens if the police really arrest the person who sent this to me.