Secrets of the crime analysts
In the 260-acre grounds of Bramshill police college, deep in the Hampshire countryside, is a quirky 1980s style building partially hidden from view.
It has several sloping roofs and is affectionately called the Sunken Pizza Hut.
In fact it is Foxley Hall and it houses one of the most secretive parts of the police service - the Serious Crime Analysis Section, Scas for short.
The little-known unit - part of the National Policing Improvement Agency - analyses the most serious sexual attacks and stranger killings in the UK to find links between them and provide police with potential lines of inquiry.
Established in 1998, Scas was briefly in the news last month, when Wiltshire Police took the unusual step of announcing it was working with the unit after finding the remains of Sian O'Callaghan and Becky Godden-Edwards.In the shadows
But most of its work is in the shadows - dealing with some of society's gravest crimes.
"It's those very serious stranger offences typically that show a serial or predatory style of offence," says Sean Sutton, the head of the unit.
"We are looking basically for those offenders who are the most serious and cause the most potential harm to the public because if they don't get stopped in their running series they're just going to carry on," he says.
Mr Sutton takes me to see the Scas analysts at work - in the basement of Foxley Hall, down a winding corridor, with surveillance cameras monitoring our every turn.
We go into a windowless office, where the analysts' desks are grouped in pods.
At one desk an analyst is staring at a pattern of dots on a computerised map - she is researching geographical links between crimes.
Nearby, one of her colleagues is poring over the details contained in a victim's police statement.
"Eighty per cent of the analysts' work is to actually understand what's gone on in the offence," Mr Sutton explains.
In a corner of the office is what Mr Sutton calls the "brain" - a computer holding the details of 16,000 sexual assaults and murders in a system called Viclas (Violent Crime Linkage Analysis).
When a new case comes in, it is analysed in minute detail. Each aspect of the offender's behaviour during the crime is then coded and the information is loaded onto the database to see whether there are similarities with other offences.
The more crimes that can be linked, the more clues there will be and the more chance detectives will have of identifying the perpetrator.
In 2007 principal analyst Theresa Jennings helped solve a series of sexual assaults in Hampshire by linking them with offences in Surrey for which a suspect had been identified.
Ms Jennings said: "There were quite a lot of distinctive features in relation to the speech that the offender used.
"They were very young females that were involved in the case... the kind of compliments paid to the victims were quite consistent between the series - that linked together with a quite a distinctive description with what he was wearing at the time."
Sometimes Scas has to work fast to support an ongoing investigation where an offender has just struck and could strike again.Identified five suspects
In December 2005 senior analyst Cath Mitchell got just such a call.
It was a case that was in the headlines - a six-year-old girl from North Tyneside had been snatched from her bath, sexually attacked and left naked in an alleyway.
Ms Mitchell examined the details of the offence and then trawled the database for similar crimes to see if any potential offenders - "nominals" - could be identified.
"We identified five nominals from our database who had committed offences either in a similar method behaviourally or they had geographic connections with the area or the offence or both," says Ms Mitchell.
One of the five nominals was a registered sex offender, Peter Voisey. He was arrested for the crime and convicted of abduction, rape and sexual assault. Voisey is now serving a life sentence.
"We were the first people to put his name into the investigation, but he subsequently came in from other sources as well," Ms Mitchell says.
"Because we'd been able to turn it round in such a short timsescale we'd been able to get that information into [the police] speedily," she adds.
Scas works closely with other units, including the Missing Persons Bureau, based in the same building, and a small team of behavioural investigative advisers.
Senior adviser Pippa Gregory says their work has changed since the 1990s when they were known as "offender profilers" and operated in a less structured way.
"Nowadays we are scientifically-based. Every report that we write, every inference we make has to be backed up with rationale, whether that's research, whether that's statistical, whether that's experiential.
"There are professional standards that we now have to work on. I think crucially as well we now have an understanding about investigations," says Ms Gregory.
She admits behavioural investigative advisers do not get it right 100 % of the time - and Scas is unable to help in every case.
Each year, the unit receives between 2,000 and 3,000 requests and is able to do detailed work in 1,500 cases. This results in about 400 positive lines of inquiry.
The case of the Night Stalker, Delroy Grant, showed that behavioural analysis work is sometimes no substitute for basic policing and detective work: Grant was caught after an intensive police surveillance operation in the area he targeted.
Nevertheless, amid the uncertainty of a changing police landscape, in which forces are having to collaborate more, Mr Sutton believes the work of his unit will be of even more value.
"When we first started I think people knew us quite well but we were still kind of 'We've done everything else; let's give them a ring'," he says.
"It's fairer to say now that most seasoned senior investigating officers will probably have one of our numbers on speed-dial rather than being the last call of choice," he adds.