It could all have been so different. Angus Sinclair was almost put on trial accused of the murders of six women. Had he been convicted, he would have been the most prolific serial killer Scotland had ever seen.

However, a decision was taken by senior figures in the Crown Office to restrict the prosecution to the World's End murders. The direct effect of that decision, which divided senior prosecutors and detectives when it was taken early last year, is that Sinclair will now never be charged with an additional four murders in which he has been identified as the chief suspect.

Sinclair, who stands 5ft 3in tall, was brought to trial at the High Court in Edinburgh accused of the rape and murder of Christine Eadie and Helen Scott after a massive inquiry, Operation Trinity, which examined a series of unsolved murders of young women in Scotland during the late 1970s.

The joint inquiry by Lothian and Borders and Strathclyde Police identified Sinclair as the chief suspect in the World's End murders of October 1977, as well as the rapes and murders of Anna Kenny in August, Hilda McAuley in October and Agnes Cooney in November of the same year. Ms Kenny, Ms Cooney and Ms McAuley all disappeared in Glasgow; in all of the cases, the victims were bound and gagged in exactly the same way.

However, when specialist criminal profilers from the FBI were asked to help out, they identified another murder in June 1977 which they were certain Sinclair had also committed. The problem was that a man had already been convicted of the murder of Frances Barker, who went missing 40 metres from the address in Maryhill where Sinclair was living. That man, who cannot be named for legal reasons, has spent the past 30 years in prison.

A senior detective at the centre of Operation Trinity said the decision not to prosecute Sinclair for the unsolved Glasgow murders was taken because the Crown wanted to avoid another embarrassing miscarriage of justice case.

The former detective said: "It was the position of Strathclyde Police that all of the cases should have been proceeded on. They had been investigated as part of a joint inquiry and all the cases were reported together to the fiscal."

The evidence that the same person was responsible for murdering the two girls in Edinburgh and four women in Glasgow was compelling.

The detective said: "We wanted to show that not only were these crimes similar, but that they were uniquely similar. It's one thing to show similarities; it's quite another to show uniqueness. In order to do that, you have to compare against everything else that's ever been. We built what is known as the Scothom database, which the FBI considers to be the most comprehensive homicide database in existence."

Detectives from Operation Trinity travelled to the US and handed over all witness statements, crime scene photographs, sketches and other materials to the FBI's behavioural analysis unit. Details of every murder case involving a young woman in Scotland since January 1969 were examined.

The conclusion of Special Agent Mark Safarik, one of the FBI's leading criminal profilers, was that not only was the same person responsible for the murders of Christine Eadie, Helen Scott, Anna Kenny, Agnes Cooney and Hilda McAuley, but also of Frances Barker.

The detective said: "We could show Sinclair was responsible because of the unique similarities in all of these cases. The FBI's behavioural analysis unit did a wonderful comparative analysis which concluded they were all murdered by the same person."

The Glasgow murders could not be prosecuted in isolation as all physical evidence had been lost or thrown out over the years. All detectives had was circumstantial, albeit compelling, evidence. Detectives in Edinburgh, however, had DNA evidence. The source said: "The evidence of identification was always going to be provided by the World's End case. That provided the identification of Sinclair. The Strathclyde cases supported the circumstantial evidence of unique similarities such as the presence of offender's signature."

But he claims there was a reluctance to revisit the Frances Barker case as it would expose a miscarriage of justice so soon after the Shirley McKie case and the Lockerbie bomber appeal. He said: "What they feared was this would have exposed the case of X the man serving a life sentence for murdering Frances Barker as a miscarriage of justice."

The decision, taken by a very senior figure in the Crown Office, to prosecute the World's End murders in isolation split the inquiry team. It is understood even a procurator-fiscal was against it.

The source said: "Frances Barker was the obstacle for the Crown proceeding because of the political implications. The scandal isn't that Sinclair has been cleared of the World's End murders, but the decision of the Crown, a political one made by a senior figure with the support of other senior Crown Office people."

Had a different decision been made the outcome could have been very different. But after the collapse of the World's End trial yesterday, the real truth may now never be known.