Murdered Melanie Hall was battered with a blunt instrument and her body wrapped in bin bags then bound with rope before being dumped by a motorway, police revealed this afternoon.

Pretty hospital worker Miss Hall, 25, from Bradford Leigh, suffered a fractured skull, cheek and jaw caused by "blunt force trauma".

Her body was then wrapped in bin liners and bound by 4mm diameter blue rope, before being buried in a shallow grave beside the M5 near Thornbury.

Detectives today revealed for the first time that they are investigating the possibility that more than one person may have been involved in her murder.

University graduate Miss Hall was last seen at a nightclub in Bath at 1.10am on June 9, 1996.

A highways worker clearing undergrowth discovered her remains last Monday on the northbound slipway at junction 14.

Speaking at the scene, Detective Superintendent Mike Courtiour, of Avon and Somerset Police, said: "The forensic examinations of Melanie's remains are still continuing and it is a slow and methodical process and needs to be so.

"What I can say is that while the Home Office Pathologist can't give a definite cause of death at this time, Melanie did suffer blunt force trauma to her head before she died.

"She had significant injuries which consisted of a fractured skull, fractured cheek bone and fractured jaw. As you are already aware her body was covered and partially wrapped in black bin bags.

"These bin bags were wrapped around her body and bound by thin blue rope.

"It is believed to be 4mm diameter polypropylene 3 strand rope and the original is now being examined forensically."

The exact site where Miss Hall's body was found was marked yesterday by two bouquets of flowers.

Two police officers were searching the area with forks and spades whilst a third methodically examined the woods nearby with a metal detector.

All earth removed from the site was transported to a table inside a large metal container where it was sifted by 10 officers searching for relevant items.

Det Supt Courtiour said his team has received 100 calls from the public since the discovery of Melanie's body and they are "actively pursuing a number of lines of enquiry".

He added that they are keeping an "open mind" as to the number of people who were involved in Melanie's murder.

"It is possible that more than one person was involved in the disappearance, murder and disposal of Melanie Hall's body," he said.

"I am once again asking for the public's help. Who inflicted serious head injuries on Melanie? Who had access to this blue rope 13 years ago?

"I am sure there is somebody out there who knows who killed Melanie then bound her body in that rope and in those bin bags.

"The search at the scene is critically important we are making sure that we look for every trace of evidence which can help us including very small items."

Miss Hall was working as a clerical officer in the orthopaedic department of the Royal United Hospital (RUH) in Bath when she vanished.

She had graduated from the University of Bath with a degree in psychology and sociology, and was having a year out while she decided what to do.

Her body was found beside the northbound sliproad at junction 14 of the M4 near Thornbury, on Monday October 5.

She was initially identified by a distinctive gold ring given to her by her grandmother and was formally identified by dental records at Flax Bourton Coroner's Court near Bristol last Wednesday.