On a rainswept morning in September 1970, the body of young mum Lorraine Jacob was found in an alley in Liverpool. She had been strangled.
The identity of her killer was a complete mystery for nearly 40 years, until retired librarian Harvey Richardson died in a hospice in 2008. Decorators charged with cleaning out his house found a detailed, nine-page handwritten note, confessing to the crime.
How many more people like Richardson are walking the streets of Britain today, free to carry on with their lives - but wracked with guilt over the dark secret they have never been able to share with their unsuspecting family and friends? How many of them wrote out a confession, only to rip it up and throw it away, never to be read.
After spending a year researching cold cases across every part of the country, I have discovered that there are certainly several hundred killers living amongst us who have never been held to account for their crimes.
I sent freedom of information requests to every police force in Britain and spent months trawling newspaper archives to compile a timeline of 1,000 unsolved murders from 1938 up until the 2020s.
Every case represents a life lost, a family shattered, and in some cases, a community destroyed.
Among these cold cases are gangland killings, contract murders, violent robberies gone wrong, street fights, and many seemingly random attacks, usually on women, carried out without reason by apparent strangers.
It is these motiveless attacks that are the most chilling. Where victims were targeted because they were vulnerable, or happened to be in the wrong place at the wrong time.
Put simply, the people who were murdered by predators who struck simply because they were fairly sure they could get away with it?
It’s impossible to say for sure. Few of the cases appear linked. But then the 1975 murder of Playgirl bunny Eve Stratford in East London had little similarity to tie it to the slaying of schoolgirl Lynne Weedon in West London later that year.
It was not until 2007 that new DNA technology showed the murders were committed by the same person, who has never been brought to justice.
Which raises the question of how many serial murderers evaded detection because they struck before advances in science enabled investigators to link their victims to the same killer.
Britain's 1,000 Unsolved Murders is available in newsagents now, and can be ordered online here
Based on his research of known serial killers, the criminologist Professor David Wilson told me he believes there are an average of two serial killers active in the UK at any given time.
But at points in the 1980s, there were as many as eight, including Harold Shipman, a family GP and respected member of his community - until it finally came to light that he was secretly murdering his patients, killing at least 215 of them before he was finally caught.
The most affecting cases in this list of 1,000 cold cases invariably involve children - such as 11-year-old Allan Graham, who was found strangled to death 24 hours after he went missing in Newcastle.
Who could do such a thing to an innocent child? What sort of monster lurks in the shadows waiting for a chance to carry out such an act of almost unspeakable evil?
One almost “forgotten” case is the Hammersmith Nude Murders. The serial killer Jack the Stripper was more prolific but far less famous than his Victorian namesake, murdering at least six women in the 1960s.
After the killings stopped, it emerged that one of the prime suspects was Harold Jones - a convicted child killer who had served time for the murder of two young girls before his release and relocation to London.
Among the names of victims in our list of 1,000 unsolved murders is one that is particularly haunting for me.
Builder John Iveson vanished in 2007. It is not known what happened to him, but two farmers were tried - and cleared - of his murder in 2012, and of disposing of his body by feeding it to pigs.
I’ve not seen John since he moved away and left my class at Brierley Street Primary School in Crewe when we were around 10-years-old.
We were not friends, but I remember him clearly at that age. It’s disturbing and deeply sad to contemplate his fate, years after I last saw him walking through the school gates.
Someone knows what happened to him.
There is - or was - someone out there who knows how and why every single person on the list of Britain’s 1,000 Unsolved Murders met their end.
And in every case, justice has been denied for far too long.