The police officer was on a routine early morning foot patrol when he spotted smoke billowing from a top-floor tenement flat.

Bravely rushing up the stairs, he managed to force his way in and called for the fire service.

The flat in Dumbarton, 20 miles from Glasgow, appeared to be empty.

However, the constable made a room-by-room search through the smoke to make sure there was no one inside.

He was about to leave when he spotted an open trapdoor into an attic space where he made a shocking discovery.

There, on a blood-soaked mattress, lay the battered body of 63-year-old security guard Eddie Cotogno.

Scattered across his corpse and the floor lay dozens of photographs of naked women.

It looked like the killer had used a hammer to beat his victim to death, such were the severity of his injuries.

The officer got on his radio and called for back-up and for the CID to attend.

He knew a murder when he saw one.

The fire had clearly been started by the killer in an attempt to hide what they had done and to destroy any incriminating evidence. 

Neighbours were shocked. 

Wee Eddie was a quiet man who kept to himself and also worked as an optical technician.

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So, who would want to kill him and why? 

Had he crossed someone in his work as a security guard? 

He'd told relatives on the morning of his death he was meeting a pal but didn't give any further details.

Eddie had been a keen photographer for years, taking portrait photographs of babies, kids on their first communion and an occasional wedding. And he had his own processing dark room in the flat.

But Eddie also had a seedy secret - he was a pornographer who took and developed nude photographs.

As police stepped up their hunt for his killer 39 years ago, the search quickly moved to Glasgow.

The women who posed nude were sometimes paid a small amount, but usually they posed for no cash reward as they saw it as an opportunity to break into modelling.

Most of them were ordinary working-class women.

Housewives, factory workers and shop assistants were happy to pay Eddie for the pics.

It was a successful business until someone decided to murder him that day on July 30, 1979. 

The murder squad quickly discovered this was more than a harmless hobby.

Eddie was in fact one of Glasgow's leading porn merchants who even made the occasional movie.

The list of motives for his murder suddenly grew much bigger.

In the porn factory he called home, detectives found numerous boxes of pictures ranging from soft porn to obscene.

Police wondered if the killer was an angry husband or boyfriend of one of Eddie's models.

Maybe one of the models had carried out the attack herself, fearing family or friends might discover her dark secret?

Maybe that was what the killer wanted the police to think?

Was the scattering of porn photos over the body a deliberate attempt to make it look as if an irate husband or woman had been the killer?

In fact, could it have been someone Eddie knew and trusted?

He also had an antiques stall at the Barras and from there he would sell the photographs under the counter to customers.

As well as his female visitors, neighbours told police a man was a regular at the flat. They knew him only as Gus.

It didn't take police long to discover Gus was in fact Angus Sinclair - who had been convicted of raping then killing his seven-year-old neighbour Catherine Reehill in St George’s Cross, Glasgow, in 1961.

Both men had become pals over their shared enthusiasm for photographing females in explicit poses.

Sinclair had been freed in 1968 after seeing seven years of a ten-year sentence for Catherine's death.

He had been friends with Eddie for some time and brought him a steady stream of women from Glasgow to photograph. 

He was also known to police for a series of brutal robberies that had taken place in the city during the 1970s

Suddenly, the 34-year-old became their prime suspect.

But he had an alibi for the night of the murder - he was at his mother-in-law's in Glasgow for dinner with his wife.

The version was confirmed by her and Sinclair's wife Sarah.

Detectives even traced some of the models, but with no witnesses, no murder weapon or forensics, the trail for the killer went cold very quickly.  

The friend he was due to meet was never traced.

He was clearly vital to the investigation and without him or any other clues the inquiry was eventually wound down.

In 1982, Sinclair was jailed for life for a series of daytime sex attacks on young girls across Glasgow, aged six to 14.

While working as a painter and decorator he would lure his victims into tenement closes and rape them.

Over the next four years he attacked at least 11 girls across the city in the same manner, though detectives suspected it was many more.

In 2001, Sinclair was given another life sentence, this time for the rape and murder of 17-year-old Mary Gallacher in Springburn in November 1978. A cold case review had identified him as the prime suspect.

Eddie's murder was filed and forgotten until almost 25 years later in 2004.

Then three Scottish police forces came together to launch Operation Trinity, a review of a series of unsolved murders of seven women and possible links to Sinclair.

Advances in DNA showed a string of cases - three in Strathclyde, two in Tayside and two in Edinburgh - could be linked. 

Officers set about building a detailed database of all unsolved female murders in Glasgow between 1968 and 1982.

They discovered that there was evidence that he may have gone on a killing spree between August and December 1977, murdering at least five women

His suspected victims were Hilda McAuley, 36, Agnes Cooney, 23, and Anna Kenny, 20, all from Glasgow, and 17-year-olds Helen Scott and Christine Eadie from Edinburgh.

All five women were found in countryside having been bound and gagged. All had gone missing after a night out at a dance hall or a pub and all appeared to have died a violent death. 

Police also took a fresh look at Eddie's murder.

They knew he and Sinclair had fallen out over the photographs they had been producing.

Eddie had taught Sinclair how to develop black and white photos of his own but he refused to show him how to develop in colour.

As a result of the 2004 review, Sinclair was charged with the murders of Helen Scott and Christine Eadie.

They had been killed following a night out at the World's End pub on Edinburgh's Royal Mile in October 1977 having been lured into Sinclair's caravanette parked in a nearby street.

Both teenagers were found dead six miles apart in the East Lothian countryside the next day. 

In 2007, Sinclair was acquitted at the High Court in Edinburgh after a judge ruled there wasn't enough evidence for a jury to consider.

However, in 2014 he became the first person in Scotland to be retried for the same crime, following a change in the double jeopardy law.

There was also new DNA evidence found on the knotted ligatures used to strangle Helen and Christine.

This time Sinclair was convicted of their murder and given yet another life sentence - his third.

However, no evidence was found linking him directly to Eddie's murder during Operation Trinity.

In 2019, Sinclair died in prison taking the secrets of the other unsolved murders to his grave.

To this day no clear motive has ever been established for Eddie Cotogno's murder.

His pornography business could have made him many enemies including those in the underworld.

Given that the pornography side-line was lucrative, could someone have wanted a share of the proceeds or stop his involvement altogether?

Or did the secret of his death lie closer to home?

Had Sinclair really killed Eddie because of an argument over processing photographs?

Or had Eddie discovered Sinclair was a monster and as a result knew too much?