The letter, found in the mailbox of a detached house on Muk Min Shan Road at about 6am on October 8, contained white powder, a teaspoon and a paper bearing a skull-and-crossbones image.
The Explosive Ordnance Disposal Bureau examined the powder and found it was not explosive. It was taken to the Government Laboratory for testing.
Police are treating the case as criminal intimidation, with a criminal investigation team following up, HK01 reported.