I have never forgotten the impression sex killer Robert Napper formed on me as he stood in the dock of the Old Bailey, guilty of slaughtering both a young mother and her helpless four-year-old daughter.

Most killers look like anyone else you'd pass in the street. But Napper was strikingly odd and there was something almost animal-like about him as he hunched up, quietly whimpering to himself, giving the impression he barely had the power of verbal comprehension.

The only comparison I could think of is Renfield, the rodent-like character in Bram Stoker's Dracula, who is so possessed by the malevolent vampire that he loses his humanity almost entirely.

I was just a year into my tenure as a court reporter at Britain's preeminent criminal court but Napper remains the most troubled - and troubling - of all the defendants I would ever cover.

A deeply disturbed loner, he had graduated from raping women in parks to sexually attacking Samantha Blissett, 27, in her home in Plumstead, south east London killing and mutilating her and then taking the life of her daughter Jazmine too.

Everyone in the court that day - even murder case veterans - couldn't help but be shocked by the horrifying details of the case: how Napper had lurked in the dark to watch Samantha at night from outside, before breaking in to attack her in the most gruesome way.

As soon as Napper's sentence was passed - he was sent to Broadmoor indefinitely - we reporters all rushed to write our stories for the next day's newspapers. But an hour or so later I was passing the same court room again, when the detective in the Napper case emerged, carrying boxes of papers.

Police officers were often ebullient in this moment after conviction - 'another case closed' - and therefore more talkative than usual. I sidled up to him.

He was, as I had hoped, quite happy to talk and gave me some good additional material on how dangerous Napper was. But what surprised me came at the very end, when I ventured to articulate out loud something I had been thinking more and more frequently over the previous few hours: 'Do you think it's possible that Napper might have killed Rachel Nickell?'

Rachel Nickell's murder was one of the most notorious of the 1990s - a young mother strolling with her two-year-old son and the family dog on Wimbledon Common set upon by a maniac and stabbed 49 times in broad daylight.

Now, thanks to a wildly popular Netflix dramatisation, The Witness, and twin documentary, as well as the Daily Mail's haunting interview with her widower and now adult son, her death in 1992 is back in the public eye once again.

The shocking attack happened in July 1992 in woodland in the south London suburb and, as both the drama tells, police soon focused on local 'weirdo' Colin Stagg.

But the case against Stagg was thrown out there before a jury was even sworn in, on 17 September 1994 after a furious judge, Mr Justice Ognall, ruled the prosecution's evidence inadmissible and strongly criticised the police for using a 'honey-trap' - a seductive undercover police officer - to try to induce a confession from Stagg.

Napper's case was in the very same court room, the historic Court One, almost exactly a year later. He even had the same defence counsel as Stagg had had, William Clegg QC.

I approached the detective that day after the details of what Napper had done were made public for the first time because it occurred to me there were obvious and immediate similarities between the murders of both Samantha and Rachel: both women resembled each other, both had worked as models, both were accompanied by young children, the extreme nature of both attacks was similar, they were both in south London.

Police officers then were more relaxed around journalists than they are today but they were still generally guarded around saying anything that could be conceived as speculative or unsubstantiated. Which is why I was amazed when this detective answered me unequivocally: 'Yes I do.'

And he went on to say that he would be talking to his detective colleagues in Wimbledon on the Nickell inquiry to tell them as much. I excitedly reported this - or tried to. 

But it was now late in the day, long after the Napper story would have been laid out to print, and if it was even published at all it would have been so deep in the story that it made little impression. Particularly when so many in the media and the wider public still believed the Wimbledon spin that Stagg was the killer.

Furthermore the story of Samantha's murder never attracted anything like the same degree of attention that Rachel's had, so Napper was carted off to Broadmoor and quite quickly forgotten.

I never forgot him though. He had been the closest thing I would ever see to a horror film character - a Psycho, in the Hitchcock rather than medical sense - and I would often think of him in the years that followed, with a shudder. 

I also never shook the belief that that detective was probably right about him. Which, of course, it would ultimately transpire, he was - Napper had killed Rachel Nickell.

But those colleagues in Wimbledon didn't listen. They still believed that Colin Stagg was the killer. And it would be another decade until advances in DNA testing would finally prove how wrong they had been.

Napper eventually pleaded guilty to the manslaughter of Rachel on the grounds of diminished responsibility in December 2008, some 13 years after Detective Superintendent Jackaman had told me about his suspicions.

Thank God he was in a secure hospital all that time so that no one else died as a result of the police failure to listen.

But that was not what happened in the notorious case that this failure inevitably reminds me of - that of the Yorkshire Ripper. 

The first team to investigate those 1970s serial killings across the north had become so convinced that taunting recorded messages sent to police from a man who became known as 'Wearside Jack' (because of his north eastern accent) were genuine that they ignored any clues that didn't fit their theory.

As a result, multiple opportunities to catch Peter Sutcliffe earlier were spurned because he had the 'wrong' accent - Yorkshire instead of Sunderland. And during this period Sutcliffe murdered three more women, Josephine Whitaker, Barbara Leach and Jacqueline Hill.

Although no one else died in a comparable way as a result of the Wimbledon fiasco, it still left Rachel's distressed family without answers for far longer than should have been necessary, something The Witness explores to moving effect.

I know now that the detective I spoke to that day in 1995 was Alan Jackaman. He has long since gone public with his frustration that those colleagues in Wimbledon had ignored his demand that they treat Napper as a serious suspect. He even wrote a book about the case, 'Napper' (2018).

He also met the actor, Steve Stamp, who played the killer in the Netflix reconstruction, The Witness, to advise him on his portrayal - which he (and I) agree was uncannily accurate.

I tracked Mr Jackaman down this week to find that he still remembered our conversation 31 years ago.

'I'm not an emotional person but I was emotional that day,' the 77-year-old recalled. 'We hadn't known that Napper was going to plead guilty and had arrived ready for a trial hence why we had those boxes of evidence.'

Mr Jackaman, who after retiring as a detective spent 13 years as a consultant with the Home Office working on cold case murders, went on: 'When we first approached the Wimbledon team looking at Rachel Nickell, we were a rag-bag team of five and they were this huge operation of 50 people and us saying 'We think you need to look at this' was just an inconvenience for them. They didn't want to know.

'Even before we had identified Robert Napper we told them we thought the two cases might be linked and their attitude was 'They can't be because Colin Stagg was in custody when Samantha Bisset was killed'. They were looking at the whole case the wrong way round.'

How, I asked him, could his fellow detectives have bought in so deeply to a theory for which ultimately there could only ever have been suggestive evidence that they were unable to open their eyes to something that, just by hearing the facts of the Napper case, had been able to see was plainly a very real possibility?

'Confirmation bias,' he said instantly. 'It's a facet of human nature to make assumptions and then be guided by them. It shouldn't have happened but it did.'

I was astonished at the time that Alan was so frank about an operational matter relating to two murder inquiries.

I now realise this was solely down to his frustration that his colleagues had refused to listen. Knowing what we do now, it's impossible to blame him.

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