Alan Wood, 50, was bound and tortured in his home in Lound, near Bourne, in October 2009.
Det Supt Stuart Morrison, who has led the investigation from the start, said he thinks about the case every day.
He said the main focus of the inquiry was to find the person whose DNA profile was left at the scene.
Det Supt Morrison said the profile was not in the UK database and officers were continuing to check international databases.
He said he had taken guidance from Professor Sir Alec Jeffreys, the pioneer of DNA profiling, as to whether it would be worth trying to trace which country the person who left the DNA at the murder scene came from.
"Whilst there are companies that will seek to tell you that information, it's not particularly reliable and it covers so many countries that it would be of no particular practical use for us," the detective told BBC Radio Lincolnshire.
Mr Wood, who ran a small gardening business, had no known enemies, according to Lincolnshire Police.
He was subjected to a "brutal and sustained" attack at his home in Lound, during which he had his throat cut a number of times and an attempt was made to decapitate him.
"I have to be optimistic that this person or people will be found. It's an investigation that I turn over in my mind every single day.
"It is something that you want to do [solve the case] for the victim and for the family, so we occupy quite a lot of our time thinking about it," Det Supt Morrison said.
Manor Lodge, where Mr Wood lived, was kept as a crime scene for two years but has recently been demolished.
The detective leading an inquiry into the brutal murder of a Lincolnshire man has said he remains determined to bring those responsible to justice.