This week's murder conviction of so-called Black Widow Dena Thompson is just the latest success for police reinvestigating unsolved crimes.

These "cold cases" are reinvestigated using a combination of scientific advances and old-fashioned determination.

Thompson may have believed she had got away with murder.

But nine years after poisoning second husband Julian Webb, the three-times-married bigamist known as the Black Widow is now finally behind bars for life after police secured a murder conviction.

"Cold case" murder squads around the country have become experts in the archaeology of crime detection - blowing the dust off old homicide files to hunt down murderers who had, until then, evaded the long arm of the law.

These detectives are experts in using advances in forensic science and old-fashioned police work to bring murderers to justice.

One of the highest-profile "cold cases" - a term Sussex Police officially frowns upon - is that of Rachel Nickell.

The 23-year-old former model was stabbed 49 times and had her throat cut in front of her two-year-old son on Wimbledon Common in July 1992.

Forensic scientists recently revealed they had found "partial DNA" on her clothing which could help track down her killer. Investigations are ongoing.

Old cases where advances in DNA technology have actually resulted in convictions include the murder of Louise Sellars in 1995, whose killer was convicted after DNA was obtained from a cigarette butt and the murder of Mary Gregson, which happened in 1977.

Scores of cold case murder squads are now springing up in forces across the country and are getting convictions where none would have been possible just five years ago.

In the ten years up to 1999, Sussex Police recorded 173 homicides - a catch-all term used by the police to refer to a murder or manslaughter. No suspects were charged in 17 of those cases.

Sussex Police's Major Crime Branch is a real-life version of the cold case squad of Waking the Dead, the BBC television drama starring Trevor Eve, Sue Johnston and Claire Goose.

Eve and his team are a small wing of the police force dedicated to reopening unsolved murder files, bringing modern forensic techniques, computer technology and psychological analysis to bear on seemingly unsolvable crimes.

The Sussex version, set up in 2001 by Detective Chief Inspector Martyn Underhill, is made up of special investigating officers and detectives of all ranks, constantly reviewing unresolved murders dating back as far as 1946.

Mr Underhill and Detective Constable Sean Scott began reviewing Julian Webb's death as a result of their involvement in Thompson's trial for the attempted murder of her third husband Richard Thompson in 2000.

He said: "Quickly we realised there were so many different angles to this case - financial, sexual, the targeting of men - so we employed a police-trained analyst to review the case.

"The analyst flagged up that Dena had been a suspect in a previous suspicious death involving another partner in 1994 in Yapton.

"While I was preparing the Richard Thompson case, I made contact with the former partner's mother, Rosemary Webb and she quickly informed me she thought Dena had murdered her son."

The investigation took a huge step forward in May 2000 when the two detectives flew to America to interview witnesses in the case of Richard Thompson.

An American called Don Hutson had written letters to Julian Webb's family following his death but had never been traced during the initial investigation.

Mr Webb, a keen fisherman, had met Mr Hutson in the Clearwater tackle shop where he worked.

The Hutsons were finally tracked down to an address 300 miles away in Orlando.

Don Hutson told the detectives he was immediately suspicious when he learnt of the death.

Thompson had told him during a telephone conversation that Julian had been fishing earlier in the day on a charter boat as a birthday present to him and had become seasick.

Thompson told Mr Hutson: "I guess the combination of the seasickness pills, the aspirin, the anti-histamines, the steroids and the alcohol was too much for his heart and he died in his sleep."

Thompson also told Mr Hutson Julian had declared he was going to eat food as hot and spicy as he could stand because it was his birthday.

Mr Underhill said: "This was crucial, because it gave another version of Julian's death given by Dena and it introduced the curry theory.

"Once we started the investigation, we went to see Rosemary Webb and raised the very delicate subject of exhuming Julian's body.

"The reason for that exhumation was the incredible advances in forensic medicine that had been revealed by the Harold Shipman case.

"Having obtained Rosemary's agreement I then liaised with the Shipman Inquiry and both coroners in Yapton and Hayling Island."

His body was exhumed in what is believed to be the first such operation carried out by Sussex Police.

Mr Underhill said: "As a result of that exhumation, Sussex Police concluded there was a real need to reinvestigate the death of Julian Webb and an incident room was set up in 2002.

"We then began to retrace the steps of what happened eight years ago."