Breakthrough technique reveals fingerprints on bullets even if they are wiped clean

It's a discovery that would make even Sherlock Holmes proud.

British scientists have developed a new crime-fighting technique that allows police to lift fingerprints from bullets even if a criminal has wiped down a shell casing.

Authorities in Britain and the United States used the method to re-open three cold cases, including a U.S. double murder that police are now optimistic of solving, said John Bond, the physicist who developed the technique.

'In one case there was enough evidence that could lead to an identification of an offender,' said Bond, a researcher at the University of Leicester and consultant at Northamptonshire Police.

The conventional method of taking fingerprints has been around for more than 100 years and involves creating a chemical reaction with the sweat left behind on an object to produce an image police can use.

But if a criminal wipes away the sweat, there is little left to react with the chemical and regular methods are useless, Bond said.

The new technique allows police to outwit a criminal and produce a fingerprint even if there is no sweat impression to work with.

The British experts focused on hair-width bits of corrosion that sweat often leaves on certain metals in bullets and bombs.

They cover the metal with a fine powder and apply a strong electrical charge that makes the dust stick to the corroded areas, producing a potential fingerprint, Bond said.

'That very fine powder only sticks to the metal where it is corroded, which means it is only sticking where the fingerprint is and means you see the image of the fingerprint,' said Bond, whose team has published its findings in the Journal of Forensic Sciences and the Journal of Applied Physics.

The technique is not foolproof and some people do not secrete enough salt in their sweat to corrode the metal to the point police can get a print, he added.

But for some seemingly dead-end cases it can provide crucial evidence and point to the person who loaded a gun used in a crime.

Detective Christopher King of the Kingsland Police Department in Georgia sought the British team's help to crack an unsolved 10-year-old double murder case and said the method had helped reignite the investigation.

'The results are surprising but to say that I am pleased would be an underestimate,' he said in a statement.

'I feel very optimistic.'