Hampshire Police has brought its 39-year investigation into the disappearance of Royal Navy sailor Simon Parkes to a close.
The 18-year-old radio operator from Bristol went missing in December 1986 while his vessel, HMS Illustrious, was moored in Gibraltar.
Mr Parkes left the aircraft carrier to go ashore but never returned to the ship.
When HMS Illustrious arrived back at Portsmouth several days later, nobody aboard knew what had become of him.
The case has now been placed on file, though detectives say they will resume their inquiries should fresh information emerge.
The investigation has long been linked to Allan Grimson, a former petty officer who was serving aboard the same aircraft carrier as the teenage sailor at the time of his disappearance.
Grimson received a life sentence at Winchester Crown Court in 2001 after being convicted of murdering two young men: Nicholas Wright, an 18-year-old naval rating from Leicestershire, and Sion Jenkins, a barman from Newbury in Berkshire.
The convicted killer has consistently denied any involvement in what happened to Mr Parkes.
The 18-year-old radio operator from Bristol went missing in December 1986 while his vessel, HMS Illustrious, was moored in Gibraltar
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Specialist officers were dispatched to Gibraltar in 2019 following what the force described as new and credible intelligence.
Detective Superintendent Adam Edwards stated: "Officers have worked tirelessly to investigate the disappearance of Simon Parkes, and to find answers for his devastated family.
"The full circumstances surrounding Simon's disappearance still remain unknown, however police will continue to support the ongoing coronial process.
"As with any cold case, no case is ever closed indefinitely – if we get new information that offers new and legitimate investigative opportunities then we will take further action."
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He explained that the intelligence received had been thoroughly examined.
Following a complex inquiry that included guidance from the Crown Prosecution Service, the force was now concluding its active investigation.
"The full circumstances surrounding Simon's disappearance still remain unknown; however, police will continue to support the ongoing coronial process," the senior detective added.
The investigation has spanned nearly four decades without establishing what befell the young sailor after he stepped ashore in the British Overseas Territory.
Det Supt Edwards urged anyone who served on HMS Illustrious and knew Parkes to contact police, particularly if they have recalled anything new since previously speaking with investigators.
"That plea also goes out to anyone who lived in Gibraltar at that time or was on holiday there on December 12, 1986," he said.
"I know it was a long time ago, but if there is anything you remember being strange or out of place, please get in touch."
Officers conducted searches at Town Range car park in August 2023.
A similar search was undertaken in January last year.
Officers conducted searches at Town Range car park in August 2023
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Previous excavations also took place at a cemetery in Gibraltar.
Senior detectives emphasised that cold cases are never permanently closed.
Hampshire Police could renew its investigation if officers discover fresh leads offering legitimate investigative opportunities.
The force continues to urge anyone who was on HMS Illustrious and knew Parkes to come forward.
The plea includes those who have not yet come forward and anyone who now remembers new information since last speaking to the police.