Years and decades pass with some of the most violent and heinous crimes in Kent remaining unsolved.

Yet there's the chance that a crucial and case-busting piece of evidence is already sitting in an archive room and holds all the secrets its owner is desperately hoping is never discovered.

Forensic expert Robert Green explains how a cold case review begins

Advances in science have opened up more possibilities than ever before to find the people to have carried out some of the most unimaginable and sickening acts humans are capable of.

Whether it is a violent and bloody murder or rape or any crime unsolved, the secrets are out there and there are diligent and dedicated teams of experts ready to pounce on the slightest thread of new information previously undiscovered.

Cold cases are some of the biggest mysteries police have to deal with and more than 50 murders remain unsolved in Kent.

In the past week, the 30th anniversary of murdered Glenda Potter in Rochester has passed and the family of Ashford teen Ashley Dighton, whose headless body was discovered in 2007, have made a fresh appeal to find his killer and the 19-year-old's head.

But how are they investigated, what are police looking for and will they ever be solved?

Robert Green OBE, a forensics expert from the University of Kent who led a programme for the Home Office setting out a national cold case strategy, says the first thing to do is to go back to the very start of the investigation.

The starting point for any cold case is to "move away from the passive approach" of appealing for witnesses and people's memories, he says.

"We advised to start with the call for service and if these have been recorded, what did they see and tell police in the first place.

"Go back to photographs at the scene and as the inquiry progressed.

"There's a saying about pictures telling a thousand words but look at those and look very methodically.

"Consider these cases being 30 years old – what couldn't we do then we can do now.

"Science has advanced massively in those 30 years and there's lots of techniques we can do now.

"You actually look at those photos and what exhibits and potential exhibits were there in that scene.

"Were they collected? If they were, were they ever submitted for what type of analysis and what result?

"Then you need to see what gaps there are in what couldn't we do then that we can do now and what exhibits were they?

"It's a start and it wouldn't be the first time things appear in photographs. The reviewing officer will look for example for a lollipop stick and whether that was recovered.

"Nowadays, we can get a very good profile from that. Cigarette ends which might have been around the scene 30 years ago there was not a lot could be done.

"But it could provide an investigative link to who might have been around that scene."

Dr Green, who is now a reader at the University of Kent and teacher of forensic science, says science can play a key role in cold cases where there is less of a link to "traditional investigative" methods.

But the key part, Dr Green says is taking a holistic and overall view of a crime scene and a case overall by taking a fresh perspective to events however long ago they took place.

Evidence such as crime scene logs, officers' pocket book notes to establish whether a witness or piece of information had been entered or followed up on.

Post-mortem reports can also provide a source of information with items historically unlikely to yield strong forensic results.

"I can recall taking umpteen finger nail scrapings from a deceased person," Dr Green, 64, from Westgate-on-Sea says.

"In those days it was a wing and a prayer and never thinking we can get anything from it but now we see finger nail scrapings form a key part of the evidence.

"So we ask were finger nail scrapings taken and were they submitted and if not why not?

"That's not a criticism, it's just saying what we could do 30 years is very different to what we can now.

"But there will be an amount of material that potentially could be looked at."

As with any investigation, Dr Green says a review should take into account science-led clues and the more traditional clues from witnesses and people.

It takes a three-strand approach – what can be seen at the scene, what was taken for testing and assessing what can be done with them scientifically today.

"That can't be done with a passive 'we are appealing for witnesses'," Dr Green says. "It requires a lot of legwork to do that."

But ultimately police budgets are smaller and reduced meaning a question of prioritising resources and efforts into cold cases remains.

Kent Police have 57 unsolved murders still on its files and while Dr Green says the force has one of the best cold case units in the country, all forces need help to see justice served to historic victims and their families.

"They are certainly equipped with the knowledge," Dr Green adds. "But over the years we have seen quite a significant impact on police funding.

"There's just that tendency to say a case is 30 years ago compared to a case yesterday and psychologically put that into a timeslot as one is more important than the other.

"But just because the offence is old doesn't mean it's an old offender and not committing offences today.

"We found repeatedly that people who commit violent sexual crimes go on to commit further offences.

"It's very often not one offence and then stop. They seem to get that urge and passion to commit that type of offence."

Kent Police say there is an absolute commitment within the force to see justice served for victims in cold cases and its dedicated cold case investigators "periodically review serious cases".

"Often we are waiting for advancement in forensic techniques, other scientific processes or new information or intelligence coming to light which supports our ability to review all relevant evidence available to solve the case," Det Ch Supt Patrick Milford of Kent Police said.

"The cold case team have been successful in solving a number of serious crimes that had gone undetected for decades.

"We are absolutely committed to seeing justice served for the victims of each unsolved murder in Kent. We do not give up and no case is ever forgotten."

"Recently Andrew Griggs was jailed for the murder of his wife Debbie in 1999 and prior to this Colin Ash-Smith was imprisoned for the murder of schoolgirl Claire Tiltman in 1993. Work by the cold case team also led to Terence Barry being jailed for the conspiracy to murder Leonard Naylor in 2001.

"Solving such cases is incredibly complex and can often take a significant amount of time, but we are dedicated to ensuring we do everything we can do to allow families to have some level of closure and to ensure these dangerous offenders are brought to justice.

"We are absolutely committed to seeing justice served for the victims of each unsolved murder in Kent. We do not give up and no case is ever forgotten."

In the case of Glenda Potter, who was found half naked, sexually assaulted and strangled in a Rochester churchyard, police in 1991 struggled to acquire much circumstantial evidence from people who knew her.

The officer brought into to lead a case review later recalled the fact Glenda was a prostitute meant few people were willing to speak to police.

Dr Green says in these types of cases the investigative approach "when people won't talk" is seeing a decline in that "pool of knowledge over the years".

Any officer who would have worked on the case will now be at least eligible for retirement or retired but could become important witnesses in their own rights.

"But if you adopt the science-led approach – these things are not mutually exclusive – it is evolving all the time," says Dr Green.

"So if you have an inquiry where traditional routes are drying up, what is that we can look at scientifically."

One of those techniques is to carry out a method called "fibre-taping" to victims' clothing.

This involves taking an adhesive dab tape over an item and picking off any fibres on the clothing.

"That would often be done with outer clothing and items of underwear. We often found scientists at the time had picked off sperm heads so not perhaps what they were looking for.

"But 30 years ago, what could you have done with that?

"It's only when you go back to look at things like microscope slides – it's cradle to grave and looking at any DNA extracts and samples taken."

A further advancement to help cold case detectives is the advent of the national DNA database which has only existed from 1995.

Now, anyone arrested must give a DNA sample to be recorded into the system.

Dr Green says often retesting items against the system can bring up results and matches for DNA profiles which had never been checked before.

"Any person found in an open scene is a much more tricky case to investigate," he adds.

"Given the lifestyles of these individuals these cases are not straightforward.

"You can draw parallels with the Suffolk Strangler victims – they had been placed in open woodland and some in water and who would have thought clues would have been found with Steve Wright.

"It really is at the edge of what can be done but I would say we should do it for Glenda's family and as importantly offenders do not just commit one offence."

Improvements to DNA and forensic science mean investigators can now work with samples smaller than a grain of sugar compared to requiring samples the size of a 50p coin 20 to 30 years ago.

While scientific advances can tell investigators something which had never been possible before, key pieces of evidence can still come simply from an untapped human source who might be holding on to information which has never been shared or disregarded as unimportant.

"One can never say never," Dr Green says. "We found things we would never have considered."

"It really is at the edge of what can be done but I would say we should do it for Glenda's family and as importantly offenders do not just commit one offence..."

But there's not a timeframe or recommended period of time for detectives to revisit cases, Dr Green says.

"It's not for us to suggest anything other than what's possible now.

"Over the years there's been a substantial reduction in funding and ability to do these things.

"A lot of things have been done when the Home Office was providing additional funding.

"But take the money away and police have finite resources and lots of current crimes to look at and therefore historic cases could be considered not to be as high on people's list of priorities."

The big question then remains is how likely is it to be that cold cases are ever solved.

"Looking back at my experiences with Operation Advance, we started with 148 samples in the freezer and were at some point destined to be destroyed.

"I took the view that they should never be destroyed. But to reanalyse these samples.

"They were DNA samples that had been taken but if you had something which predated that, you could go back to the exhibit itself if you have it.

"From those samples we found a third of them matching straightaway – on the DNA database – just literally sitting in freezers.

"They had been taken predating the database and had been taken but weren't compatible.

"This is where the development of all this knowledge and experience came from looking at all of these cases.

"We looked at offending patterns – had they committed one offence and stopped?

"No, they hadn't. They continued offending into late adulthood.

"By looking at these old offences we can potentially prevent crimes of today."

Dr Green said it had not been his experience working with police forces of failing to investigate crimes fully where victims had been sex workers.

"It wasn't our experience in Operation Advance either.

"Let's call these girls street workers, they don't make false allegations and have a right to have the police investigate.

"Investigatively, it's not been put into second place based on who the victim was.

"Given everything about the difficulties of investigating these cases 30 years on, there is still a will in the service to do this.

"That has to be balanced against today's cases and slimming resources.

"These things are never forgotten and will be in the archives of evidence that could potentially open the case.

"Any form of fibre-taping, nail scraping and clothing could be relooked at but that requires time and money."

Even with the advances in science to develop a DNA profile there can be issues when there is no match provided.

"These things are never forgotten and will be in the archives of evidence that could potentially open the case..."

Dr Green explains the process of familial searching where detectives are able to get a match based on a relative – parent, sibling or child – committing a crime and their record being entered into the DNA database can provide a link to historic cases from archive held evidence.

"The case of Joseph Kappen who had been murdering in the 70s and cases were linked biologically pre-DNA and a profile was recovered from clothing and body swabs – not too dissimilar to Glenda's case.

"It matches the victims but there was no match on the database.

"But it did match the son of the offender who had gone on to commit crime.

"The father died and his son bore a striking similarity to what we found on the bodies.

"It's at the edge of what we can do but certainly worth having a go at.

"Kappen was a posthumous conviction."

Genealogy exploration services where people can submit DNA to find out more about their ancestry are also providing more DNA samples to analyse than ever before.

It allows scientists to explore links between cousins which Dr Green says means the potential well of evidence is "never over".

"But it all depends on what was collected, if it wasn't collected, it's gone.

"Those items that were but are in the exhibit system somewhere could be the clue.

"This knowledge and experience ought not to be forgotten.

"Who would have thought we would be doing this sort of thing back in the 80s."

To get the latest updates in ongoing cases, police appeals and criminals put behind bars, click here

To read more of our in-depth features click here.

Read more: All the latest news from Kent