THEY are the unseen crime fighters who rely on science to succeed where traditional police work cannot.
Working with test tubes and computers, they hunt for the silent witness, the unseen clues, which can unravel a case which has baffled hundreds of officers.
With the conviction this week of Peter Tobin for the murder of Vicky Hamilton 17 years ago, the work of the experts at Scotland's national forensic science service has been thrust out of the laboratory and into the spotlight.
Today, the scientists, who work for the Scottish Police Services Authority (SPSA), lifted the lid on how they helped solve one of Lothian's longest running cases.
They pieced together the damning evidence that convicted Tobin, with experts in fingerprints, DNA profiling and toxicology all drafted in to sift through evidence nearly two decades old and make a case stick.
It was this expertise which allowed binbags, buried in the garden of Tobin's former home in Margate and containing the 15-year-old's body, to still yield his prints.
Tests for drugs in an Edinburgh laboratory were able to prove Vicky had taken anti-depressants, Tobin's favoured sedative on previous victims. And with DNA technology, scientists showed that the teenager's purse, discovered in St Andrew's Square 11 days after she vanished, had been handled by Tobin's three-year-old son.
Their work, often carried out with a fallibility of one in a billion, sealed Tobin's fate and ensured he was found guilty.
It was the discovery of Vicky's purse near Edinburgh's bus station on February 21, 1991, which strengthened a police theory that she was a runaway, leading to two fruitless months searching for her in London. But the false lead planted by Tobin unwittingly proved part of his undoing as techniques that would only evolve years later linked it back to him.
Nicola Clayson, 39, was one of a team of biologists at the SPSA's laboratories in Howdenhall Road involved with collating the DNA evidence.
She said: "From swabs taken from the purse, we were able to extract three DNA profiles. We used a blood spot sample taken from when Vicky was a newborn baby to confirm one was hers.
"But the strongest profile was from Daniel Wilson, Tobin's son. I believed it might have been the strongest because he may have touched the purse with his mouth."
The DNA traces were detected on the purse on April 30 last year, four days before Tobin was convicted of raping and killing Polish student Angelika Kluk.
The find spurred police to strip and search 11 Robertson Avenue in Bathgate where Tobin lived at the time of Vicky's disappearance.
Ms Clayson, who worked at the Metropolitan Police labs in London before moving to the Capital 12 years ago, helped conduct a sweep of the address, removing possible samples for testing.
After a knife was founded concealed in the loft, SPSA biologists were able to prove by June 15 that skin recovered from the blade matched Vicky's DNA.
The investigation moved onto another former Tobin residence in Margate, Kent, where Vicky's remains were dug up in binbags on November 14 last year.
Peter Sibbald, an SPSA fingerprint expert based at Fettes police HQ, was asked to prove that Tobin's prints were on the 17-year-old bags. He said: "The scene examination branch in Glasgow used a metal deposition process where gold and zinc was applied in a vacuum to the bags. This process is very sensitive, but detects the fatty constituents from sweat on older prints.
"A print can last a long time if kept dry and clean. Here, a number of bags had been used on top of one another to store the body so we were able to use one of the inner bags. The soil was full of clay and sand which offers good drainage, keeping the bags protected and the prints fresh."
The 54-year-old, who joined Lothian and Borders Police as a constable in 1978 before transferring to the fingerprint unit in 1983, said: "Prints were taken from Tobin while he was in Peterhead Prison. From the bag, he had prints for a right little finger, left ring finger and a right middle finger. Using a holistic identification standard we confirmed the prints came from Tobin."
Two postmortems were carried out on Vicky's body, one in Kent and the other at the city mortuary in the Cowgate.
Dr Martyn Okely, 61, who works for the National Centre for Criminal Toxicology, also based at the SPSA lab in Howdenhall, used the samples taken to search for drugs from the 17-year-old remains. He said: "We looked for signs of drugs of abuse which are amphetamines, benzodiaxepines, like valium or diazepam, cannabis, cocaine, methadone, methylamphetamine, which shows up ecstasy, morphine and opiates, such as codeine or dehydrocodeine.
But we did not find anything with these tests.
"We then searched for prescription and non-prescription drugs. Here we found traces of amitriptyline and nortriptyline, prescription anti-depressants which can have the side-effect of sedatives.
"From medical records, we confirmed that Vicky Hamilton was never prescribed these drugs. But Peter Tobin was."
Lothian and Borders Police have come under criticism after the Evening News revealed how detectives in Portsmouth alerted them to Tobin's ties to Bathgate in 1993 while they hunted him over sex attacks on teenage girls.
Many of the procedures used by the forensic scientists, such as obtaining fingerprints from the buried bags and identifying the presence of drugs in Vicky's remains, would have been available at that time to build a case. Others, like the DNA profiling, are the products of the latest technological breakthroughs.
THE SCIENTIFIC TRAIL ..
A – THE GRAVE
FOUR clear fingerprints were recovered from the black wrapping of binbags that Peter Tobin used to bury Vicky Hamilton in Margate.
A metal deposition machine in Glasgow was able to lift sweat from one of the inner bags and produce photographs of legible prints.
Edinburgh fingerprint expert Peter Sibbald said four of the ten photographs produced were "very good", making it "practically infallible" that they did not belong to Tobin.
B – THE DRUGS
TOXICOLOGISTS carried out a raft of tests to show that anti-depressants once prescribed to Tobin remained in Vicky's body.
A biochemical technique called Enzyme-Linked Immuno-Sorbent Assay, or ELISA, proved the drugs were present.
Mass spectrometer, as well as gas and liquid chromatography readings, were taken to confirm beyond doubt that the drug's chemical fingerprints were present.
C – THE PURSE
KEPT in a police evidence room for 17 years, Vicky's purse was linked to Tobin's three-year-old son, Daniel, by his DNA profile.
Police visited Daniel, now 21, to ask for swabs after Strathclyde Police told their Lothian colleagues about Tobin's Bathgate links following the murder of Angelika Kluk.
Despite the passage of time, they proved an exact match with a one-in-a-billion chance of being wrong.
Forensic biologist Nicola Clayson said: "If a sample is kept dry and not exposed to damage, DNA can be preserved for many years. Blood can last 50 years."
D – THE KNIFE
A TINY scrap of tissue was recovered from a knife found hidden in the loft of Tobin's former home in Bathgate where a jumble of his old belongings still remained.
The SPSA's labs in Howdenhall were also able to prove that DNA taken from a blood sample of Vicky's when she was only hours old matched the sample on the blade.