Detectives are "one important step" closer to finding the killer of Melanie Hall thanks to DNA evidence from where her remains were found.
Hall, a university graduate, was last seen at 1.10am on June 9 1996 sitting on a stool on the edge of the dance floor at Cadillacs nightclub in Bath.
A workman found her remains in a bag in vegetation next to the northbound slipway of junction 14 on the M5 at Thornbury on October 5 2009.
Detective Superintendent Andy Bevan, of Avon and Somerset Police, revealed the DNA evidence to mark the 20th anniversary of Hall's disappearance.
Crimestoppers is reinstating a reward of up to £10,000 for information leading to the arrest and conviction of those responsible for the murder.
"For the first time I'm able to confirm publicly that we do have DNA evidence which was left on an item found at the scene where Melanie's remains were discovered," Bevan said.
"Through new techniques, we're in the process of developing a DNA profile. There's no doubt this brings us an important step closer to finding her killer."
The force does not have a prime suspect or any "conclusive evidence" that Hall's death is linked to other murders or sexual offences.
"I believe the DNA profile we're developing will assist these inquiries significantly," Bevan added.
Nine people have been arrested in connection with the case since the investigation began but no-one has been charged.
Police are asking for sightings of Hall after 1.10am on June 9 1996, including of the university clerical worker leaving the nightclub.
Witnesses to suspicious activity or a vehicle parked near the spot where the remains were found are asked to come forward.
Hall's clothing and belongings have never been found, including her pale silk dress, black suede mule shoes, a cream jacket and handbag.
Her parents, Steve and Pat Hall, said their lives had been "totally altered" by Hall's death and pleaded for anyone with information to come forward.
"There are a number of people there and we just need one of them to come forward with that key bit of information," Hall said.
"What they don't appreciate is there's not just one victim in this situation, there are a number of victims.
"There's my wife and myself, my daughter, 97-year-old mother - my mother. Our lives have been totally altered and they can't be put back together completely.
"We would simply say, this is what someone has done to us, please see fit to let us know what happened to Melanie and who was responsible."
Hall said his family felt a "responsibility" to find his daughter's killer on her behalf and bring them to justice.
"It is the last act that we can carry out for her," he added.
The Operation Denmark Incident Room can be called on 0117 952 9788, while Crimestoppers can be contacted anonymously on 0800 555111.