ADVANCES in DNA technology have revolutionised the investigation of crime and the administration of justice.

New techniques including the ability to analyse literally microscopic amounts of blood have prompted police forces throughout the country to review hundreds of unsolved murders in the hope of a breakthrough. DNA analysis is an extraordinarily powerful tool in unlocking compelling evidence. Like any new technology, however, it is limited by human failing and the case against Ross Monaghan, who was acquitted last week of the murder of Kevin "Gerbil" Carroll raises some disturbing questions.

One problem with DNA evidence is the danger of cross-contamination. Police officers who searched Mr Monaghan's home seized a jacket found to have a single particle of firearms discharge residue. However, they were wearing the uniforms they had worn earlier on a firearms training exercise, so their clothing was covered with bullet residue. Such oversights should simply not occur in a police force with trained officers and a properly accountable chain of command.

Alarm bells must ring even more loudly, however, over the relationship between expert forensic staff at the Scottish Police Services Authority (SPSA) and investigating police officers. The court heard evidence from one forensic expert that she had prepared a report stating that the residue particle was of a similar type to that used in cartridges recovered from the crime scene but that this conclusion had been reached at the request of a detective superintendent involved in the investigation.

Forensic evidence has a powerful sway because lay jurors are not in a position to challenge it. Unless the findings are arrived at scientifically and without pressure, they lack validity. It is understandable that the police are keen to find evidence that strengthens their case. But a major strength of the forensic service is its independence. In the Carroll case, the judge Lord Brailsford dismissed the single particle as of no evidential value. That should act as a warning to every investigation officer.

It is right, however, that the Home Office Forensic Science Regulator also conducts a full and independent investigation, not least because, despite the exposure of an over-close relationship between police and forensic services in the Shirley McKie fingerprint investigation, it appears the importance of maintaining independence and professional distance has not been fully appreciated. Any form of interference risks a miscarriage of justice.