Una Crown murder: Crimewatch reconstruction knife appeal
An 86-year-old woman was murdered for £40 cash and her engagement ring, police believe.
Una Crown was found stabbed to death in her bungalow in Magazine Lane, Wisbech, Cambridgeshire, on 13 January.
Her body had been burned in a bid to destroy evidence but the house had not been ransacked.
After a reconstruction of the murder on BBC's Crimewatch police said a 10ins (25cm) kitchen knife was a vital clue they needed to find.
Mrs Crown was described as regular in her habits, always doing her shopping on Friday helped by her niece and nephew, Judy and John Payne.
When she paid for her shopping she always asked for £40 cashback, police said.
After she was dropped home on Friday 11 January, the only other contact known to police by Mrs Crown was a telephone call to a neighbour a day later at 17:00 GMT.
She was described as "very security conscious" and always "locked herself in her house when she was alone there", detectives said.
Mrs Crown had been stabbed repeatedly in the chest and injuries to her hand indicate that she tried to defend herself.
The £40 cash cannot be accounted for and the only thing missing from the house was an engagement ring she had worn for 60 years.
Det Insp David Grierson said: "We do not think the knife came from the bungalow. The attacker brought it with him.
"From her injuries we were able to establish it was probably a long bladed kitchen knife 10ins (25cm) long.
"We have not found it and we need to."
He appealed on Crimewatch for "that vital piece of information or evidence that tells us who was responsible for this".
Mr Grierson said after the BBC programme: "We received a number of interesting calls following the reconstruction which we will be following up over the next few days.
"However, I would continue to urge anyone with any information about Una Crown's murder to call police."
A £10,000 reward has been offered by independent charity Crimestoppers for information that leads to the arrest and conviction of Mrs Crown's killer.
A 58-year-old man arrested in late January in connection with the investigation remains on bail until 15 May.