DETECTIVES interviewed 4,500 people, took 300 sets of fingerprints and spent 45,000 man-hours hunting the killer of housewife Ann Heron.

The person leading the investigation changed a number of times, but the unstinting desire to solve the apparently motiveless murder was constant.

Successive senior detectives kept a photograph of a smiling Mrs Heron on their desk at work to provide the inspiration for them not to give up on finding her killer.

For Keith Readman, the detective superintendent who led the inquiry for five years, the baffling case from the summer of 1990 became his obsession.

There were appeals on the BBC1 programme Crimewatch and on Tyne Tees Crimestoppers, but the breakthrough never came - despite a typed tip-off naming potential suspects.

Less than a year after the murder, three Darlington men were arrested in connection with the killing, but were later released without being charged.

Mrs Heron's husband, Peter, once put up a £500 reward for information leading to his wife's killer, and in the years that followed her death, he spoke to The Northern Echo several times.

In 1995, he told the newspaper: "I believe I know my wife's killer," saying he was a local man, someone he had rubbed shoulders with every day.

Mr Readman handed the unsolved investigation - and Mrs Heron's smiling photograph - over to Detective Superintendent John Blake in 1995.

"Looking back," he said, "all I can say is I did my best, but unfortunately it was not enough."

Despite his disappointment, Mr Readman remained adamant that the murderer would one day be caught, and said he looked forward to receiving the phone call to tell him the news.

In the summer of 1996, police visited an unnamed sex offender in connection with the killing, but the interview came to nothing.

Eight months later, convicted rapist Philip Hann was moved from Wakefield Prison to Darlington to answer questions relating to the case, but he, too, was quickly ruled out.

Four years later, detectives said they would quiz convicted killer Philip Smith but, like all the other leads before, this one was soon dropped.

On the 11th anniversary of the death, Mr Blake announced that a breakthrough could be "just around the corner" and the key to the case would be science.

When he retired in 2003, the photograph of Mrs Heron was passed again to Chief Superintendent David Jones, who brought the full weight of technology to bear on the case.

In the past 18 months, police have told of several major developments in the case.

The Northern Echo revealed last July that detectives were carrying out laboratory tests using the latest forensic technology, which could bring them tantalisingly close to finding the killer.

Last November's arrest of Mr Heron was the biggest development in the case, but where yesterday's revelation that the case against him was being discontinued leaves the investigation remains to be seen.

But a Durham Police statement from Chief Supt Jones last night revealed: "We have commissioned a full review, by independent experts, of all the forensic evidence available to the investigating team. It is expected the work will take several months to complete.

"We remain determined to do everything we can to bring this investigation to a successful conclusion.

"I am hoping a fresh look at the evidence by some of the country's foremost experts, who until now have had no involvement in our inquiry, will yield new information that will take the case forward."