Traces of DNA have been found on a length of rope wrapped around bin liners containing the body of Melanie Hall, police have revealed.
Detectives said it was “possible” that the DNA could belong to the killer of Hall.
The 25-year-old, a clerical worker at the Royal United Hospital in Bath, was last seen alive in Cadillacs nightclub in the city at 1:10am on June 9, 1996.
Her remains were discovered next to the junction 14 northbound slip road of the M5 by workmen cutting grass on October 5, 2009.
There have been 11 arrests during the investigation into Hall’s murder, but no-one has ever been charged.
Detective Chief Inspector James Riccio, of Avon and Somerset Police, said officers had previously revealed that DNA had been recovered from an item found by Hall’s body.
“Today, we’re confirming the DNA traces were found on a length of blue polypropylene rope which was wrapped around thin black bin liners which had contained her body,” he said.
“We recovered a 13-metre length of 4mm rope from the scene, made up from four separate lengths knotted together.
“In addition to the three knots joining the ropes, there were four other knots on the rope – a total of seven knots.
“It’s commercially manufactured rope and commonly used on building sites and for drawing electrical cable through trunking.”
When asked if the DNA traces could be those of Hall’s killer, Riccio replied: “It is possible”.
He described the DNA as being “in its embryonic stages”, but said advances in technology were being developed every day.
“We’re confident that, with corroborative evidence, that will lead to a charge,” he said.
Police believe that the area where Hall’s remains were discovered is “key”.
Her body would have been left “in haste” in the undergrowth either on the morning of June 9, 1996, or within a few days of that date, they say.
It is likely the person would have then driven onto the northbound M5 motorway and either onto the next junction, or have turned off at Michaelwood services to head back in a southerly direction.
“It’s quite an odd place to dump a body,” Riccio said.
“People know how Melanie got here. People know how Melanie died.”
He said it was “highly likely” that the person who left Hall’s body in the undergrowth was familiar with the area.
The person who did this may not have killed her, he said.
In a direct appeal, Riccio urged that person to come forward to police for the sake of Hall’s family.
“You’ve been living with a dark secret for years, but your guilt and fear is nothing compared to the enduring pain felt by Melanie’s family,” he said.
“I want to be clear – our primary focus is on identifying the person or persons who killed Melanie.”
In a statement, Hall’s father Steve Hall said it was 23 years since their youngest daughter was murdered.
“Since that time the Avon and Somerset Police have poured endless resources in their attempt to find her killer,” Hall said.
“Sadly, as yet, this objective remains unfulfilled, although I and my family remain eternally optimistic that eventually they will be successful.
“In our family we will forever grieve for, and miss our lovely daughter. A young woman whose life stretched before her until that fateful night in June 1996, when that life was so cruelly snatched from her.
“She will never fulfil her life’s ambitions, never marry, never have children and my wife and I will never have another grandchild.”
Police are asking for anyone who owned a length or lengths of blue polypropylene rope that went missing around June 1996, or knew someone who had rope of this kind and whose behaviour had changed since that date to come forward.
They want to hear from those who were involved in the aftermath of Hall’s murder, including assisting with leaving her body in the undergrowth.
Anyone who saw Hall after 1:10am on June 9 or saw her leave the nightclub should contact police.
Detectives also want to hear from anyone who saw suspicious activity or a vehicle parked on the northbound junction 14 slip road of the M5 at Thornbury in the early hours of June 9 or days after.
Hall’s pale blue silk dress, black suede shoes, cream jacket and black handbag have never been found, along with cosmetics, her cheque book and bank card.
Her jewellery including a Next watch with expanding bracelet and a pair of silver drop earrings are also missing.
Anyone with information should call the Operation Denmark incident room on 01278 648934 or give information online.
A force spokesman said: “Melanie’s parents have reinstated a £50,000 reward for information provided to the police which leads to the arrest and conviction of the person or people responsible for Melanie’s murder.
“The independent charity Crimestoppers is also reinstating a reward of up to £10,000 for information leading to the arrest and conviction of the person or persons responsible for the murder of Melanie Hall.
“You will only qualify for this reward if the information is passed to Crimestoppers direct.”
To date, Avon and Somerset Police has received 1,751 calls from the public about the case and taken 1,668 statements.