Frank McPhie was killed by a single shot fired by a sniper who hid in high flats opposite his home in Glasgow on May 10, 2000. The hitman left the murder weapon at the vantage point but police were unable to catch the killer and McPhie's murderer has never been traced.
Detectives insist the hunt is still ongoing and have asked for anyone with information about the circumstances of the killing to contact them.
A spokeswoman for Police Scotland said: "As this is an unresolved case, it remains open.
"We would appeal to anyone with any information to get in touch with the police."
The renewed appeal came in response to research carried out by a leading criminologist into contract killings.
Professor David Wilson and his team - which included investigative journalist Donal MacIntyre - interviewed McPhie's former neighbours and analysed newspaper archives and court transcripts. They also spoke to convicted killers as part of their research.
They examined 35 hits - 27 murders and eight non-fatal shootings - since 1974, and discovered that the average cost of a contract killing in the UK is £15,000.
Professor Wilson, who lectures in criminology at Birmingham City University, and his colleagues also found that the cheapest hitman was paid just £200 while the most expensive was alleged to have been paid £100,000. Their case files showed that guns are the weapon of choice for most assassins, and were used on 25 of the 35 victims.
Glasgow gangster McPhie, 51, was murdered outside his home in Guthrie Street, Maryhill, minutes after he had had an argument at an allotment he owned near his home. He received a single shot to his head as he walked towards the door of the tenement block.
The fatal hit came from a drying area on the eighth floor of a block of flats opposite McPhie's home.
The hitman left the rifle at the vantage point. DNA was later discovered on the weapon, but after its analysis experts discovered it belonged to a police forensic specialist.
Officers discovered McPhie made several phone calls as he drove home, and one detective said this suggested the mobster "was maybe trying to rally some people".
The officer added: "It made perfect sense that some altercation was created to force McPhie into the trap".
Police hit a wall of silence during their investigation into McPhie's murder, but five months into the inquiry suspicion fell on underworld figure John McCabe, 51.
McCabe - a key lieutenant in the notorious Daniel crime clan, who were locked in a feud with McPhie at the time - was arrested over the murder. But the procurator-fiscal dropped the charges through lack of evidence and McCabe was never prosecuted.
At one stage police thought the assassin might have been an Ulster terrorist, but sources in Glasgow have claimed the trigger was in fact pulled by an ex-soldier from the north of the city.
In the 1990s, McPhie - who had been imprisoned for various crimes - was jailed for eight years for his part in a £200,000 drug deal. He was also cleared of two murders after both juries returned not proven verdicts.
Professor Wilson added: "The conversations I had with McPhie's old neighbours were off the record but the majority were relieved he was dead."