A dismembered finger has been posted through a letterbox in Bognor Regis, police said.
The tip of an unidentified male digit was sent to an address in the West Sussex town on September 10.
The DNA profile has been checked against the national database but a match has not been found.
Officers are checking CCTV following the "isolated and unusual" event, and have also made inquiries with hospitals in the hope of establishing the person's identity.
However, they are confident that this was not a targeted incident.
Sussex Police's Chichester and Arun District Commander, Chief Inspector Jon Carter said:
"While we accept it may be alarming, I would like to reassure people that there have been no reports of similar incidents in the area, and at this time there is no evidence to suggest that this is part of criminal activity, or any person has been targeted."
