Alan Holmes, a 53 year-old garage hand who worked at Kentish Town Police station, was tied up and left to die at his home in Camden, north London.
He was dropped off by friends at his flat in Parkway at around midnight on Christmas Day 1995 but failed to report to work over the following week.
It was only on 4 January 1996 that police officers forced entry and found him tied hand and foot to his bed, face down and fully dressed.
Although still alive, he was critically ill due to being restrained in the same position and dehydration.
Before his death, he managed to tell officers that he had been left tied up since Boxing Day – although one of his killers had returned to the flat following the attack.
Alan died on 5 January 1996 from blood clotting due to his injuries.
Police believe that his killers tortured him into handing over two Natwest bank cards and his PIN numbers before leaving him to die.
CCTV cameras recorded a total of £1,000 being withdrawn but the images have not helped to identify any of the suspects.
One original line of enquiry was that the attack was carried out by members of the homeless community.
Making an appeal for witnesses in December 2010, DCI John Oldham of the Homicide and Serious Crime Command said: “It is now 15 years since Alan was killed and those responsible have never been held to account.
“We hope that another year on, our re-appeals reach someone who may have information that will help the enquiry. I hope they now feel that they can come forward and speak to police.”
Contact detectives on 020 8785 8244 or Crimestoppers anonymously on 0800 555 111.