Joanna Yeates, whose snow-covered body was found by dog walkers on Christmas Day, was strangled, police said today.
Detective Chief Inspector Phil Jones of Avon and Somerset Police revealed the results of a post mortem examination as he declared that her death was now officially being treated as a murder investigation.
He said Joanna, who disappeared on Friday, December 17, had died as a result of compression of the neck.
He appealed for any information about her murder, adding: "Somebody out there does know what happened to Joanna."
Speaking at a press conference he said police believed her body had been at the spot where she was found for several days.
"What we have to do next is to try and find out why she was killed and who was responsible," he added.
The 25-year-old landscape architect had been missing for eight days when her clothed body was discovered on Longwood Lane, close to Bristol and Clifton Golf Club, in Failand, North Somerset, about three miles from her home.
The frozen condition of her corpse meant pathologists could not immediately examine it - leaving detectives and family members with an agonising wait to find out her cause of death.
Miss Yeates, from Clifton, Bristol, was last seen alive on December 17 when CCTV showed her in a Tesco store on her way home to her flat in Canynge Road.
Miss Yeates's body was formally identified by parents David, 63, and Theresa, 58, who yesterday made an emotional visit to the site where she was discovered.
The distraught pair, who were joined by a small clutch of family members that included their son Chris and their daughter's boyfriend Greg Reardon, 27, laid a bouquet of yellow roses with a picture of their daughter on her graduation at the scene.
Forensic investigators have been examining the site since the tragic discovery in an attempt to find clues about her suspicious death.
Detectives are also examining CCTV footage from Clifton suspension bridge in Bristol, which is on the main route from Miss Yeates's house to where she was found.
The last known sighting of Miss Yeates was on CCTV as she bought a pizza at a branch of Tesco Express in the Clifton area on the evening of December 17.
Miss Yeates, who worked at BDP with Mr Reardon, was making her way home from the Ram pub in Park Street at about 8pm after a night out with work colleagues.
On her way home she rang her best friend Rebecca Scott to arrange to meet on Christmas Eve.
Miss Yeates lived with her boyfriend, who reported her missing on Sunday night after returning home from a weekend away in Sheffield visiting family.
Officers released footage of Miss Yeates buying the pizza in Tesco, which has become a key part of the investigation.
There was no trace of the pizza, the wrapping or the box in her home - despite the fact that the receipt, the coat she was wearing, and her mobile phone and keys were inside.
Anyone who can help the investigation can call the Operation Braid incident room on 0845 456 7000 or Crimestoppers on 0800 555 111.
Questioned by reporters, DCI Jones said Mr Reardon was being treated as a witness not a suspect.
His laptop and mobile telephone have been examined by detectives but officers have stressed that this was a routine procedure in such inquiries.
DCI Jones said police were pursuing various lines of inquiry into why Miss Yeates was murdered.