A reward of up to £20,000 has been offered to help find the killer of Jia Ashton in Derbyshire.
Mrs Ashton's body was found in Sleetmoor Woods near Somercotes on 13 March. She had been assaulted.
The 25-year-old economics graduate is thought to have been killed after leaving work at the nearby Thornton's factory on 10 March.
Police said an anonymous benefactor had offered the reward for information that led to a conviction.
Det Ch Supt Steve Cotterill, of Derbyshire Police, said: "We have made tremendous progress in the investigation so far and I'm grateful to all the people who have called us.
"However, we are still missing some vital pieces of information which will help us identify the killer.
"I hope the offer of this reward will encourage others who have not yet come forward to do so."
Detectives said the focus of the inquiry was now very much on the identification of the man seen in the woods after Mrs Ashton left work at Thornton's.
He is described as about 6ft tall, of athletic build, between 45 and 55 years old and with blond or mousey stubble on his chin.
There was no evidence a weapon had been used in the attack and there was no apparent sexual assault, police said.