The two officers tasked with finding Arthur Brumhill's killer in 1993 and 2013 have expressed their sadness having never managed to bring his killer to justice.
Speaking outside Northampton Crown Court yesterday, former Chief Superintendent Bob Thorogood said the tragic death at Pal Denton Pet and Garden Supplies in 1993 remains one of his biggest regrets.
The lead investigating officer in the case 24 years ago wrote a book about how the Mr Brumhill’s killer had escaped the net and yesterday watched from the public gallery as former store employee Stuart Jenkins was acquitted of the 76-year-old's murder.
It means the case, which was reopened back in 2013 and given fresh impetus by the discovery of a set of fingerprints on an abandoned bag of straw, will now remain on file as unsolved.
Mr Thorogood, now 70 and retired, said: "Our thoughts go out to Susan, Arthur’s daughter who is not very well at the moment.
"I'm just sorry we weren't able to get a positive result.
"But it was a matter for a jury.
"The jury had a doubt - and if they had a doubt they had to find him not guilty."
Mr Thorogood said the very nature of the crime scene, "red herring" sitings of a suspicious looking man in the area back in 1993 and the absence of DNA evidence all combined to make it a perplexing case, which may now never be solved.
He said: "There were a number of difficulties and the pet shop itself was the first problem.
"There were pets, plants, food all over the place. To examine the scene was a massive task.
"The forensic scientists were in there for five days, which was unheard of. That was in addition to the police searching the premises."
One witness reported seeing two boys running off along Monks Pond Street, opposite the pet store, on the night of the murder.
They were never found, and the crucial statement has haunted Mr Thorogood ever since. Even though the boys may have had nothing to answer for, it would have ruled out another line of enquiry.
"We didn't track the boys, but it wasn't for a lack of trying," he said.
"There was a great deal of trying. We just had to work on the evidence we could gather."
The case was reopened in 2013 as part of a periodical review of cold cases and taken on by Detective Chief Inspector Louise Hemingway.
In 2015, fingerprints found on a ripped bag of straw led the force to arrest former shop employee Mr Jenkins.
The defendant's barrister, however, successfully argued the prosecution's case was "flimsy" and former army sergeant Mr Jenkins was acquitted unanimously yesterday.
DCI Hemingway said she was "disappointed for Mr Brumhill's family" outside of court yesterday.
She said: "This case was resurrected after 20 years. They never had any expectation of a conviction before that.
"But then we got this breakthrough and we brought it to court, there was hope. We always knew it was going to be tough because it was a historic case."
However the detective has vowed to keep searching for Mr Brumhill's killer.
"We will do what we did before," she said.
"We will look at advances in science or any other advances in the evidence.”
But she conceded: "We have taken it as far as we can at this stage."