Producer, BBC News
The Sion Jenkins case ultimately turned on 158 tiny spots of Billie-Jo's blood found on her foster father's clothing.
Sion Jenkins attended the third trial with his wife Christina
Too small to be seen with the naked eye, they have been scrutinised over the years by leading experts in blood pattern analysis from around the world.
The jury was asked to decide whether the pattern of blood spatter proved Mr Jenkins was the killer.
The defence argued it was caused by a fine spray of blood as he tended Billie-Jo after the attack.
The focus of the police investigation changed from the moment these microscopic blood spots were discovered on Mr Jenkins' fleece, trousers and shoes in the days following Billie-Jo's death.
"As soon as there was a scientific opinion that seemed to suggest that Sion Jenkins was guilty, all proper policing ended," said Jenkins' solicitor, Neil O'May.
"That was a disaster in trying to find who killed Billie Jo and it was a disaster for the criminal justice system".
Forensic scientists told police they believed the pattern of the blood was the result of "impact spatter".
They determined from the shape and position of the drops that they must have got there as Mr Jenkins was standing over his foster daughter and hitting her repeatedly.
Their evidence led to Mr Jenkins being charged and, in 1998, convicted of the murder.
But his defence team had already begun looking for an alternative explanation for the presence of the blood.
They turned to Professor David Denison, one of the country's leading experts in lung disease at the Royal Brompton Hospital in London.
He found evidence of what he believed was a rare condition called pulmonary interstitial emphysema, or PIE.
This meant there was a build-up of pressure in Billie-Jo's lungs, which Professor Denison claimed happened in the minutes after the attack, before she died.
He believed this was caused by blood blocking her airways, and that some of the blood was released after Mr Jenkins moved her, sending a spray of droplets which landed on his clothing - called "expiration spatter".
Prof Denison reconstructed the conditions of the murder - this time taking into account the pressure in Billie-Jo's lungs - and created a similar pattern of blood.
"My experiments show that you can generate from the mouth and nose the sort of spatters that were found on Sion Jenkins' clothing and the distribution is almost identical - the size of the droplets is almost identical - so it is a very, very credible explanation," said Professor Denison.
As a result of this evidence, three Appeal Court judges quashed the murder conviction in 2004 and ordered a retrial.
Since then, nearly 30 scientists from around the world have been consulted.
One of these is Joe Slemko, a Canadian Mountie who gained much of his experience from analysing blood at crime scenes.
"It's important to look at where the blood is, but also where there is not blood," he told the BBC.
"The tell-tale blood stain pattern that caught my eye was the pattern that was on the chest area of the jacket. It was a condensed, confused pattern and that could only have been created as a result of an expiration event. "
Blood pattern analysis has been an important tool for forensic scientists for more than 40 years, but this inquiry has pushed the technology further than ever before.
158 spots of Billie-Jo's blood were found on Mr Jenkins' clothes
Professor Jim Fraser, a past president of the Forensic Science Society, said it showed how different explanations can often be found for this type of evidence.
"It's good, sound, reliable evidence or information," he said.
"But you can interpret the information in different ways so any interpretation of blood on clothing or at the crime scene is highly dependent on what you know about the crime scene or what you can infer with that typical type of crime."
Professor Graham Zellick, head of the Criminal Cases Review Commission that sent the Jenkins case back to the Court of Appeal, said that while he did not want to criticise the latest jury, it raised wider issues for the role of juries in the future.
"The jury may well be the ideal, or at least the best mechanism we have for deciding who's telling the truth," he said.
"But to decide detailed technical matters on which some of the world's leading experts are arguing, well that strikes me as being fanciful."
Police say hundreds of people have been convicted partly on the basis of blood pattern analysis in the past.
The verdict shows how difficult it is for jurors to assess conflicting expert evidence.