A serial killer could be responsible for the multiple murders of pensioners, according to a confidential report leaked to the press.
The Sunday Times claimed the report raises concerns two suspected double murders in Wilmslow, Cheshire, could have been the work of an offender unknown to the police.
The first two double killings happened in 1996 and 1999 only two miles apart. In both cases husbands were previously thought to have murdered their wives before killing themselves.
But after re-examining the cases using modern techniques, the report finds both cases were likely to have been double murders, reports The Sunday Times.
Cheshire Police said they were conducting a review of the findings in the report, which was handed to the force last month, the newspaper said.
They are also said to have alerted Greater Manchester and Cumbria Police – where some of the killings took place.
The 179-page report calls on the National Crime Agency and Interpol to review cases in Britain and Europe to determine whether there are more related murders.
"This individual will not stop killing until someone or something stops him," the report is quoted as saying.
The report, written by Stephanie Davies, the senior coroner's officer for Cheshire, is supported by evidence from her predecessor at the time of the first deaths and a US-based crime-scene analysis expert.
The deaths of Howard and Bea Ainsworth and Donald and Auriel Ward were considered by police to be murder-suicides.
Both couples were discovered lying on blood-soaked beds in their nightclothes.
Similarities between the cases included the extremity of violence, with knives left in bodies at the crime scene; injuries to the head from a blunt weapon and stab wounds; and the fact that the women had been left with their nightdresses lifted.
The report points to "a number of inconsistencies which do not corroborate the original manner of death of being murder suicide".
It also identifies a further three murder-suicide cases, in 2000, 2008 and 2011 which should be reviewed, two in Greater Manchester and one in the Lake District.
In each case police said the wife had been stabbed and hit on the head with a blunt object by the husband before he took his own life.
A spokesman for Cheshire Police told The Sunday Times: "We are in receipt of the report and it is being reviewed. This is a piece of research which has been undertaken by the staff member, independently.
"As with any case that has been closed, where new information comes to light it is reviewed and acted upon if appropriate. We have notified Greater Manchester Police and Cumbria constabulary."