Another victim who is believed to have been killed by the murderer of missing nightclubber Sian O'Callaghan went missing "several years ago", detectives revealed today.
Police said they had launched a painstaking search at a new location in Gloucestershire after confirming Miss O'Callaghan's body has been recovered from an area near the village of Uffington, 15 miles east of Swindon.
Steve Fulcher, who is leading the double murder inquiry for Wiltshire Police, said there was "only one" outstanding missing person on the force's records.
Officers said they had been searching a site in Northleach, across the county border, following information from the suspect, named locally as taxi driver Christopher Halliwell, who told them the locations of both Miss O'Callaghan's and another victim's body.
Detective Superintendent Fulcher said: "Searches of that area continued through the night. It is a painstaking and slow process."
In a frank admission, he later added: "We do not at this stage know what we are dealing with."
He said officers were seeking more time to question murder suspect.
Describing the ongoing search for a second body, he added: "What I can say is that if there is one, it relates to an incident some years ago."
He said officers had only one person registered as missing, but added: "It is not appropriate for me to reveal any detail."
Chief Superintendent Steve Hedley paid tribute to public support after what has been a "very difficult week".
Officers had been searching near the famous "White Horse" chalk figure in rural Oxfordshire after arresting Halliwell outside an Asda supermarket.
Police seized a green taxi which was seen early on Saturday when Miss O'Callaghan disappeared after leaving a nightclub in her home town of Swindon.
Officers began searching a home in the town's Ashbury Avenue, erecting a white tent in the front garden of the semi-detached house.
Specialist dog teams have been helping the search after mobile phone technology pinpointed several areas of interest in 4,500-acre Savernake Forest.
Miss O'Callaghan, an office administrator, was caught on CCTV leaving the nightclub, in Swindon's Old Town area, after an evening out with girl friends.
She lived in a flat just half a mile away with her boyfriend Kevin Reape, 25.
She had sent a text message to her partner on the night of her disappearance, saying: "Where are you? x."
Police revealed earlier this week that phone records put Miss O'Callaghan's mobile somewhere in the forest 34 minutes after she left Suju nightclub at 2.52am on Saturday.
Miss O'Callaghan's family have thanked members of the public who have helped in the search for "our beautiful girl".
Shortly after 10am a convoy of police vehicles went through the cordon at the site where Miss O'Callaghan's body was found, close to the Uffington White Horse.
The six or seven-strong cavalcade included police cars and vans, plus an unmarked van towing a white trailer.
Police are likely to make an application "within the coming hours" to magistrates in a bid to extend the suspect's detention.
Mr Fulcher added: "He currently remains in custody here at Swindon.
"During the course of dealing with him I was personally taken to two locations. One near Uffington where we discovered the body of a woman who we believe is Sian, and also to another location near Northleach where I believe a second body may have been taken.
"I believe that Sian's body was moved to the Uffington area by this individual. Sian's family have been fully informed through the night including the fact that Sian's body has now been recovered."
Post-mortem examinations are ongoing on Miss O'Callaghan's body, he said.
He added: "This is a tragic situation, I have spoken to the family ... our thoughts are with Sian and her family."
He said it was not a "simple" exercise to ascertain whether there is a second body at Northleach.
"I can tell you that Wiltshire only has one outstanding missing adult and that is from several years ago."
Officers refused to discuss reports they had been in contact with police in Somerset regarding the 1996 disappearance of Melanie Hall. Avon and Somerset police also had no comment.
Police have set up roadblocks on five roads around the Gloucestershire villages of Eastleach Turville and Eastleach Martin, about 10 miles (16km) north of Swindon.
Police investigating Miss O'Callaghan's death are using mechanical diggers to excavate the corner of a field on the Oxfordshire-Gloucestershire border in their hunt for a second body.
Around 15 officers and forensics experts are searching land at Baxter's Farm, between the villages of Eastleach and Filkins.
They have removed part of a dry stone wall and have excavated two trenches in the corner of the field, where officers in protective clothing were on their hands and knees in the mud.
DCI Sean Memory, who is leading the search, said officers had received "relatively specific" information from a man held in connection with 22-year old Miss O'Callaghan's death but that nothing had yet been found.
"We were given some indication of where there might be a find. Of course, it isn't as simple as that. It was tricky last night in the dark. We will use as much light as we can and then decide what we will do when it gets dark," he said.
Police have erected a blue tent in the field where they are searching.
DCI Memory said they had been searching since last night using powerful lights.
The farm at the centre of the search lies within rolling countryside a couple of miles south from the Cotswold Wildlife Park, outside the small market town of Burford.