The body of Juozas Meilunas was found in a Kettering flat on Boxing Day.
The 51-year-old, who was originally from Lithuania, was found in the flat in Woodlands Court, off Wood Street, Kettering, after reports of a flood.
A day later police revealed he had been murdered, and that he had been dead for some time before being discovered.
Neighbours spoke of their shock that Mr Meilunas' body could have gone undiscovered for so long, while detectives appealed for anyone with information to contact them.
If you can help the police with their investigation call 101 or independent charity Crimestoppers anonymously on 0800 555 111.