Four ways to catch a killer

Dirty footprints on a carpet could be a killer piece of evidence. Different makes and styles of trainer have their own tread patterns and the police now have access to databases that reveal whether the attacker was wearing Nike Air Max or Converse All Stars. And the way the tread has worn or any damage to the sole may also help to link a footprint at the murder scene to a suspect – a kind of fingerprint for feet.

The police will have to work quickly, though. Each day the attacker roams the streets a free man (or woman), they will subtly change the wear on the tread. So three months is regarded as the maximum length between a footprint being left and a suspect’s trainers being apprehended for analysis. Dr Sarah Jacob analyses footprints for the Forensic Science Service, which does forensic work for police forces across England and Wales. “The important thing to remember with footwear is that it has the potential for a conclusive link,” says Jacob, “because if you’ve got random features on a shoe – cuts and gouges – which are reproduced in your scene mark, you can eliminate all other shoes of that type.”

If you don’t have a suspect, the tread pattern could still help the police to narrow their search. A study published last year by Dr John Bond, head of forensic science at Northamptonshire Police, revealed that the type of trainers a criminal wears speaks volumes about them. After analysing data from over a year’s worth of burglaries in Northamptonshire, Bond and his team found that criminals from the most deprived areas wear the most expensive trainers. Not only that, but the younger they are, the more expensive their trainers, and those who are unemployed tend to furnish themselves with the most costly kicks of all. The research is already being put to good use in Northamptonshire and it’s hoped it will spread to other police forces.

For years, it was thought impossible to take prints from discarded bullet casings. Traditional fingerprint methods rely on the presence of sweat, which leaves a mark. But the intense heat of firing a gun vaporises any print left – so your evidence simply evaporates.

Bond approached the problem from a new angle. Sweat, he realised, might not be present after a bullet has been fired, but while it is there, it corrodes the surface of brass. “We found the interface between the corroded metal on the surface and the uncorroded metal underneath actually forms a diode, in the same way as you have diodes in electronics,” says Bond. “It conducts electricity in only one direction.”

It’s this property that Bond’s clever method exploits. First a 2500-volt electrostatic charge is applied to the casing, then it’s dusted with fine carbon powder. The diode effect means the corroded areas aren’t as good at conducting, so their charge is less and this tends to attract the powder, revealing the fingerprint.

If a gunman’s been eating lots of meals-for-one before their attack, they will have made themselves easier to track. Processed foods tend to be high in salt – and the higher salt content of the sweat corrodes the metal more.

This shattered glass could put a suspect behind bars. If you can prove the tiny fragments of glass found on their sweater match the glass at the crime scene, you’ve got them bang to rights. And the amounts of glass involved are tiny – fragments of 0.25-1 millimetres are typically what the police find on a suspect’s clothing and hair.

So how do you know whether the glass on the sweater is the same as the glass on the carpet? A good place to start is by comparing their refractive index. This is the amount by which glass bends light that travels through it. Because of differences in the manufacturing temperatures, glass from different sources will have a different index. If it’s the same, you have a match.

Measuring the refractive index is fairly simple. The glass fragment is mounted in oil and the two are heated. As the oil gets warmer, its refractive index changes and at some point it will become the same as that of the glass – the glass will disappear from view under a microscope. As you know the index of the oil at different temperatures, you can work out the index of the glass.

The composition of the glass can also be analysed under an electron microscope to find out whether there’s a match. “You’ve got different glass compositions depending on what it’s used for,” says Sarah Jacob of the Forensic Science Service. “Manufacturers want them to have different properties so they add different trace elements like magnesium or aluminium.”

Our victim has a bite mark on his leg. It looks fairly fresh, so it could well be linked to the crime. These days criminals – particularly gang members – are using dogs as a weapon because it’s thought to be more and more risky to carry guns or knives (although in this case, they clearly thought a gun was worth the risk).

“Gangs are increasingly using dogs to control their victims,” says Angela Gallop, director of science and innovation at LGC Forensics, another company that provides services to the police. “If you’ve got a gang dog then it’s like a piece of equipment and that piece of equipment can leave traces of itself indirectly on victims.”

In the past, even knowing whether hairs left at a crime scene came from a cat or dog was difficult – they look surprisingly similar under a microscope. But recent advances mean that forensic scientists can distinguish between cat and dog hair and may even be able to pin down the breed of dog from the DNA.“Our experts, using different DNA tests, can now confirm or exclude a specific suspect dog as the source of hairs found,” says Gallop, who is setting up a new unit at LGC focusing on animal forensics. “Of course animal DNA can be found in other traces aside from hair, like saliva in bite marks in clothing.”