Hanratty's family claim exhibits were contaminated

The damning DNA evidence that ties Hanratty to the murder of Michael Gregsten and the rape and shooting of his mistress, Valerie Storie, was discovered on two exhibits.

The first was the female victim's underwear; the second was the handkerchief found wrapped around the murder weapon, a 38-calibre revolver.

Forensic experts interviewed for a special Horizon programme on BBC Two say the genetic profiles recovered from Exhibits 26 and 35, as they were logged at the original trial, come from one man - James Hanratty.

'Beyond doubt'

The chances that the DNA came from someone else are millions to one, they say.

Lord Woolf, the Lord Chief Justice, who with two colleagues considered the posthumous appeal, said the DNA evidence established Hanratty's guilt "beyond doubt".

Hanratty's body was exhumed for DNA testing

So why, even after the appeal, do his family still maintain his innocence?

Simply, it comes down to the possibility of contamination of the exhibits.

The technique of genetic profiling was not invented until 1985 - two decades after the crime was committed on the A6 at Deadman's Hill in Bedfordshire.

And it is only in the last few years that the technique has been sufficiently well developed to get a readable profile from the minute biological samples involved in this case.

Unforseen advances

Investigators in the 1960s could not have foreseen such astonishing scientific advances.

Consequently they would not have imposed the strict handling guidelines used today to protect items taken from a crime scene from possible cross-contamination.

Geoffrey Bindman said contamination was likely

Indeed, during Hanratty's trial at Bedford Assizes, the exhibits were routinely taken to and from court in the same boxes.

Geoffrey Bindman, the Hanratty family lawyer, told the BBC: "We know that exhibits at the trial in 1961, including Hanratty's own clothing, were mixed together.

"Witnesses handled his clothing and other materials found at the scene - contamination could very easily have taken place.

"Exhibits were handled freely because people did not know about DNA."

Evidence magnified

Scientists use something called the polymerase chain reaction (PCR) to amplify a DNA sample.

The process will "unzip" the double-stranded molecule and make two exact copies.

By repeating the process, scientists can generate vast quantities to analyse.

DNA tests have unlocked key evidence

The minute samples of DNA extracted from the Hanratty exhibits were subjected to more than 30 separate cycles of PCR magnification.

When the DNA was analysed it showed both items had come into contact with the same man.

When this profile was then compared with the one built up from Hanratty's own exhumed remains (from material taken from his teeth) - again, there was a perfect match.

So what of cross-contamination? Could skin cells, for example, from some of Hanratty's own clothing have been transferred to crucial exhibits? Was this the DNA that the PCR amplified?

Forensic scientists who worked on the case say this is highly unlikely - and in any case, they found no other profile. If James Hanratty was not the killer, then where was the killer's DNA?

The Hanratty campaign believe the case has merely shown up the limitations of genetic fingerprinting.

Unfortunately for them, the judges at the Court of Appeal were far more impressed with the science.

They considered the cross-contamination issue and dismissed it.

Lord Woolf said: "In our judgment... the DNA evidence establishes beyond doubt that James Hanratty was the murderer."

Horizon: The A6 Murder can be seen on BBC Two on Thursday, 16 May, at 2100 BST.