The younger brother of a man murdered 35 years ago said he will continue campaigning as he cannot break a "promise" he made to his sibling.
Peter Miller, 24, was stabbed to death in Great Yarmouth on 9 December 1984 and no-one has ever been charged.
The forensic potential of the case has been limited as 170 case exhibits were destroyed, returned or lost.
Mr Miller's brother, Tony, said he had to remain "hopeful" the case would be solved but admitted he had "doubts".
He said: "Sometimes I think what am I doing? Why am I wasting my life? It all comes back to the same thing - I made a promise, he was my brother, that's what I should be doing."
Mr Miller was found stabbed on the kitchen floor of the family home in Camden Place, having last been seen by a neighbour in the afternoon.
Fresh information in 2013 led to a "sharp implement" being uncovered and officers re-appealed to find a woman who called Great Yarmouth police station on 30 December.
Despite the woman never being traced, Det Insp Neil Stewart said he continued to focus on her.
"Thirty-five years later I hope the lady is still about and hopefully she's got the confidence now to call us again and give us that information," he said.
A recent review of material from the national forensic archive failed to find a breakthrough and Det Insp Stewart said "clearly it's not going to help" that there was a lack of exhibits at his disposal.
"The important thing for me is we have a good relationship with Tony Miller," he said.
"He is very passionate about and committed to finding out who killed his brother and I share that commitment."
Mr Miller previously called the police investigation "flawed" and that they had "stolen" justice from the family.
He said "there's nothing more important in my life" than finding justice for his brother.
"I made a promise to my brother and that's not the sort of promise you break.
"My brother Peter would have done exactly the same for me."