In gruesome confessions the former Royal Navy petty officer told detectives how he viciously attacked 18-year-old naval rating Nicholas Wright and 20-year-old barman Sion Jenkins after luring them to his Portsmouth flat.
Killed a year apart on December 12 in 1997 and 1998, the two men’s deaths landed cruel Grimson with a 22-year jail term.
But questions remain over the disappearance of naval rating Simon Parkes, who vanished during shore leave from HMS Illustrious on December 12 in 1986.
Grimson, 60, who was serving on the warship, was among the last people to see Mr Parkes alive. He has always denied killing the 18-year-old radio operator.
Did Allan Grimson kill missing HMS Illustrious sailor?
Links between Grimson, formerly of London Road in Portsmouth, and Mr Parkes are now the subject of TV documentary Murdertown due to be screened on the Crime+Investigation channel on Monday.
The thought of Grimson’s possible Parole Board bid to leave prison, when he becomes eligible to apply in November, fills naval rating Mr Parkes’ mother, 73-year-old Margaret Parkes, with a sense of dread.
‘That’s the worst thing possible to think he’s walking around free,’ she tells The News from her Bristol home, ‘not because of his crimes but because I think he’s a very dangerous man.
‘I don’t think psychologically he can change even after all these years.
‘He’s a very clever man, he can use a situation. When they were profiling him at the time (for a previous BBC documentary) the programme had to be very, very careful - he’s very manipulative.’
Frankenstein thought killing was 'better than sex’
Nicknamed Frankenstein by his naval colleagues, Grimson served 22 years in the senior service travelling the globe. But he was secretly a murderer who would later tell police killing was ‘better than sex’.
Grimson had previously taught Nicholas Wright on a firefighting course and the pair were seen talking at the back stairs of Joanna’s nightclub at Southsea seafront on December 12, 1997.
Retired detective Neill Cunningham, whose interviews elicited confessions from the killer, said: ‘For years it had been popular with the Royal Navy at the end of a night out and then that same night, his friends talked about – they may have seen him speaking on the back stairs of Joanna’s nightclub, to the firefighting instructor they’d previously met on a course, and that was the last time anybody saw of him that evening.
‘He never reappeared. His family reported him missing to the navy and he was dealt with as AWOL, which is absent without leave, which in effect is quite a major difference to how you deal with a missing person.
‘But not uncommon to go absent without leave in the navy. It’s not necessarily that they’d always report it to the police even now.’
Missing persons case led to two confessions
When Hampshire police were informed of the disappearance a missing person probe was launched.
That led to witness statements being taken, including Grimson’s own statement - where he admitted being on the stairs with young Nicholas.
Thought of as ‘funny, helpful, just an all-round nice guy’, Grimson was brought in for questioning over Nicholas’ death. Left overnight to stew in custody, detectives hoped a second day of questioning might be more fruitful than the first.
It was then he confessed. ‘They sat on the sofa at the flat,’ Mr Cunningham tells viewers in the documentary, ‘Allan explained that he made advances on him, tried to kiss Nicholas and was pushed back, and it just made him angry. Really angry.
Hiding his homosexuality in the navy
‘And this side of his character came out. He’d been hiding his homosexuality within the services for a long time and then… just beat Nicholas.
‘And then he got him in the hallway and he remembers at one point Nicholas was laid on his front and Grimson was on his back and he had a diver’s knife; a really sharp diver’s knife, and then with his hand he started motioning how he cut him and what he did.
‘And then he put him in the bath and went to bed. He talked about that he spent all day with Nicholas in the bath and what was he gonna do with the body? How am I gonna get rid of the body?’
All of this was around 18 months since Nicholas had gone missing. Grimson led police to the body, near Winchester, which would otherwise never have been found.
Confessions to two killings – but denial of involvement in Simon Parkes’ death
Asked by officers if there was anything else, Grimson said: ‘There’s one more body.’
Grimson confessed to murdering Sion Jenkins, barman at the Hog’s Head in Palmerston Road, Southsea.
‘He invited him back to the flat because Allan wanted to get that feeling of power and control again,’ said Mr Cunningham.
‘A feeling that is so difficult to get and so he took Sean back, and Sean was really drunk, and he basically held him hostage for hours at knifepoint.
Attack on second victim was quicker
‘With Nicholas it was quicker because Nicholas pushed his advances away, but Sean was more submissive.
‘He assaulted him, eventually he put Sean into the bathroom and then bashed him with a baseball bat a number of times.’
Investigators trawled through details of missing sailors who served with Grimson. Simon Parkes’ case stood out when a naval officer present at the interview pointed out the rating vanished on December 12, now a significant date, in 1986.
The teenagers’ parents, primary school teacher Margaret and chemical industry worker David, 77, were shocked - but grateful - when police contacted them about Grimson’s possible involvement.
Mrs Parkes said: ‘We didn’t want to hear that at all. It would have brought closure (had Grimson admitted any involvement).
‘But it was just the fact that something was being done that someone was investigating it and that was an amazing feeling to know that we weren’t alone anymore.’
Serial killer in nature
Grimson admitted killing Mr Wright and Mr Jenkins. The judge who jailed him for 22 years said he was a ‘serial killer in name if not number’.
Searches were carried out in Gibraltar by police in January 2003. A body was found, but it was not that of the missing sailor.
Mrs Parkes added: ‘By now we are 100 per cent (of Grimson’s involvement). Simon’s remains have to be found, or Allan Grimson has to admit that he killed him.’
Grimson denies being involved and complained about a BBC documentary and Crimewatch episode. Ofcom ruled against him.
A Hampshire police spokeswoman said: ‘As with all cold cases, it is regularly reviewed as part of routine work carried out by our teams to see if new information or new developments in technology can progress the case any further.’