Specialist tracker dogs have been called in to help in the Abigail Witchalls case, more than a month after the young mother was stabbed. What, if anything, might they find after so long?
At work at the crime scene
In Paul Auster's novel Timbuktu, a dog's ability to smell is indulged by its owner concocting a mixture of the richest smelling things he can devise.
"A dog had roughly 220 million scent receptors, whereas a man had but five million, and with as great a disparity as that, it was logical to assume that the world perceived by a dog was quite different from the one perceived by a man," Auster wrote.
But there are sniffer dogs and there are sniffer dogs, it seems.
Some six weeks after the vicious knife attack on Abigail Witchalls, Surrey police have called on the services of four sniffer dogs with exceptional abilities.
Even your average pet dog is blessed with an outstanding nose compared with humans. A dog's sense of smell is tens of times better than ours and this, coupled with the fact humans are pretty poor at covering up their scent, explains why dogs are important in helping to investigate a crime scene.
But sniffer dogs come in all shapes and sizes - any breed can be trained - and have a variety of skills. Cadaver dogs, for example, are specialists at searching for corpses; tracking dogs can locate a specific person on the basis of scent.
The nose knows
Despite the fact dogs were used to search the scene of the attack immediately after it happened, the new canine recruits - two from the Dyfed Powys force, two from South Yorkshire - have more refined abilities.
Police have collected scent from a number of suspects, and implanted these on pads for the dogs. Having learned the smells they're looking for, the dogs are roaming specific parts of the woodland around Little Bookham, Surrey, where the attack took place.
Abigail Witchalls was left for dead
"We are looking for the weapon and any items the offender may have discarded," says Detective Chief Inspector Brian Russell of Surrey police.
But humans don't just leave their scent on what they carry and wear. The thousands of microscopic skin cells we constantly shed provide a trace of where we've been.
"[The dogs] can work through brush and undergrowth and the scent still permeates from those areas," says Mark Harrison, a national police search adviser involved in the case.
Sgt John Cod, of Dyfed Powys police, says its two dogs - a German and Belgium Shepherd - will comb areas outlined by Surrey police, looking for body fluids.
They will be accompanied by a Border Collie and a Springer Spaniel from South Yorkshire; the springer is specifically trained to sniff out human blood.
Tony Cronin, whose company Dragon K9 trains search dogs, says the process is straight-forward.
"You put the scent on a tennis ball and throw it around for the dog to retrieve. Then you progress to hiding the scent itself and reward the dog with the tennis ball when he's found it."
It takes about three months to get standard search dogs up to scratch, he says.
When they are fully trained, dogs exhibit quite remarkable powers of detection, he says. A dog could detect five grams of cannabis in a room the size of five-a-side football pitch in three to four seconds.
But the training of these super-sniffers is far more intense, says Sgt Cod, and has pushed the boundaries of what tracker dogs can do.
"We've opened out the ability of these dogs and experimented with what they are capable to do with their nasal capabilities. We know now that what we've asked them to do in the past [was] restricting their ability."