Corinna Marr murder: Adelaide model was shot dead execution-style 27 years ago - and her killer got away with murder. Now a friend has revealed the 25-year-old's big secret - and it blows the case wide open

The unsolved murder of a young model has come under renewed scrutiny after a close friend made a bombshell new claim that she was having affairs.

On the afternoon of July 4, 1997, Adelaide woman Corinna Marr, 25, was shot dead execution-style - with bullets to her head and chest - in her suburban unit. Her killer has never been brought to justice.

There was no sign of forced entry, and the way Ms Marr was murdered suggested she was comfortable with whoever was in her home that day.

The real estate worker and part-time model's close friend and boss Colin Todd has now claimed that she told him she was having affairs before her death.

'I was giving her advice as a friend, a close friend, in regards to certain activities that she was involved in with affairs with other people,' Mr Todd told Channel 9's Under Investigation program on Tuesday night.

'And I actually indicated, "You're doing situations too close to home with people you shouldn't".'

Mr Todd was on the way to pick up Ms Marr to take her to a job that evening, when he called ahead to say he was running late.

He was told by Ms Marr's husband Robert that she was dead. 

Channel Nine's Liz Hayes put it to Mr Todd that he was under suspicion at the time. 

'I arrived at the scene and I knew that anyone that's at a murder scene early in the piece becomes a suspect,' he replied. 

'You know, you've got to put yourself in that boat.' 

He said the police interviewed him later that afternoon and accused him of killing Ms Marr. 

Hayes asked him straight out if he killed Corrina.  

'No, I didn't,' Mr Todd replied.

Daily Mail Australia does not suggest that Mr Todd or Mr Marr was involved in anyway in Ms Marr's death.

Almost three decades after his daughter was murdered, Ms Marr's now elderly father Trevor King is still looking for answers. 

'Who did it? Why did they do it? You don't take a beautiful girl out of the equation,' he said.

'There's no reason for it to happen. She wasn't in any criminal activity or anything like that.

'We just want closure really. Hopefully something will come up. Hopefully before I die,' the heartbroken Mr King said.

From 2004 to 2012, barrister Paul Heywood-Smith KC, who was representing a never-charged alleged suspect in the case, was not allowed access to secret documents in the police file on the case.

'And as a result of that, we don't know a lot,' Mr Heywood-Smith said.

Police argued that revealing the information 'was not in the public interest' - which Under Investigation said could mean they did not want to reveal their methods or interfere with the continuing investigation.

'We are now 27 years after the murder. Police methodology would have changed so much ... that any methodologies back (then) will be of no interest,' Mr Heywood-Smith said.

Former Adelaide news executive Graham Archer said that 'in an open investigation, police are entitled to say, 'We don't want to release this information.'

'But in saying that, I would've thought it was in the police's interest to demonstrate that they were following other leads,' he said.

'I happen to know, speaking to senior police, that they actually are concerned about this case but when I asked whether the (suppression) would be lifted, they said that will never happen.'

The special investigation into the execution of Corinna Marr can be watched on 9Now.