A £10,000 reward is being offered to encourage people to come forward with information about the murder of a 74-year-old man in his home.
David Brickwood was attacked at the property in Lindsay Avenue, Abington, Northampton, in the early hours of Saturday September 26 and later died in Northampton General Hospital.
Northamptonshire Police said Mr Brickwood had been the victim of a "violent assault".
Officers said one of the key lines of investigation is two Ford Focus cars which were seen being driven closely together in the area at the time.
Detective Chief Inspector Mick Graham, who is leading the investigation, said: "Mr Brickwood's family are clearly still devastated by the way he died and I would appeal for people who have information about who may be involved to look to their conscience and come forward to the police.
"I would particularly ask family members who may have seen relatives acting oddly the day after the murder, or who came home with blood on their clothing, to consider if this was your father or grandfather who had died, how you would feel - and I would appeal for them to come forward."
Anyone with information is asked to contact Crimestoppers anonymously on 0800 555 111.