Show me the evidence

Detectives can now recreate crime scenes down to the tiniest detail. Jimmy Le Shreeve meets the laser-scanning crimebusters

Police are using sophisticated laser-scanning techniques to create virtual reconstructions of crime scenes, helping detectives to solve difficult cases and juries to make more informed decisions.

Police are using sophisticated laser-scanning techniques to create virtual reconstructions of crime scenes, helping detectives to solve difficult cases and juries to make more informed decisions.

The American television show CSI: Crime Scene Investigation has been criticised by real-life investigators for being too far-fetched, stretching the facts as well as the science. Of course, investigating crime scenes still involves much painstaking "plod" work. But it is becoming decidedly hi-tech, especially in complex cases.

One of the key technologies used today by police is laser scanning, a technique that creates accurate three-dimensional reconstructions of crime scenes. These reconstructions can be viewed on large computer monitors and can even be explored from different angles and points of view - in a similar manner to computer games.

Understanding evidence documented on a two-dimensional drawing of a staircase is difficult. "If you create a 3D staircase, the relevance is often clear," explains the laser scanning specialist Derry Long of Plowman Craven and Associates (PCA), a land surveying firm, which also has police authorities among its clients.

Long - who spent 12 years as a civilian employee of the Metropolitan Police - has set up Europe's first hi-tech call-out team for criminal investigations. He and his colleagues are always on call, and they are responding to more incidents every year. Their job involves scanning the area where a crime has been committed and then recreating it, down to sub-millimetre level, using digital data along with standard photographic documentation and notes from the crime scene.

The detail offered by 3D laser scanning can make all the difference to solving puzzling aspects of a crime. In one case, reported in the US Government Technology magazine, a murder was thought to have occurred in a kitchen, but no one could work out how the body ended up in the hallway. By recreating the crime scene, police were able to examine the area from different points of view and work out what happened.

Laser scanning is particularly useful when a crime has been committed outdoors. It speeds up the mapping of very large crime scenes, and allows the investigators to glean far more detail. When an execution was carried out in a large pasture in Ireland, investigators used laser scanning to create a virtual map of the scene, and quickly gained vital information they wouldn't have gathered with traditional methods. In the past, the sheer size of the area would have required protracted surveying (using a theodolite), sketching and photographing.

"The beauty of laser scanning is that it can capture enormous scenes down to the minutest level," says Mark Harrison, National Police Search Adviser and one of the officers involved in the investigation in Ireland. Harrison first used 3D laser scanning in February 2002, when faced with finding bodies after the Yarl's Wood detention centre for asylum-seekers, in Bedfordshire, burnt down. It was the largest building-based crime scene in Europe.

"There was concern that bodies might have been in the burnt-out building, so a forensic deconstruction of the site had to be performed," he says. "The great difficulty was how to capture such a massive crime scene."

Another advantage of laser scanning is that it minimises the possibility of crime scene contamination. "This can take many forms," Harrison says. "Someone might touch an object, leaving their fingerprints. Or they might inadvertently move or take evidence from the scene, possibly by picking up hairs on their shoes. The great thing about laser scanning is that it is non-intrusive. It lets you capture a scene from a stand-off point of view, greatly reducing the contamination risk."

Harrison also points out that scanning a crime scene digitally before anyone enters is also a way of cementing the scene in stone. When a case comes to court, this can make a big difference to a witness's recollection of events. "From an evidential perspective, it's brilliant because you can pinpoint where everything was," says Harrison. "It's also interactive. You can put a digital representation of a witness into the 3D model and move them around the scene. This can help to verify what they could or couldn't have seen. It might turn out, for example, that they couldn't have seen an incident as clearly as they thought they could because an item of street furniture was in their direct line of sight."

Laser scanning works equally well in documenting accident scenes. Although he's not involved personally in this, Harrison points out that the technology was used earlier this year by British police to re-examine the Paris tunnel where Diana, Princess of Wales died in the car crash in 1997.

The virtual magic of 3D laser scanning has also been used to create visual effects in movies such as Tomb Raider and the Harry Potter series. But, for some people, the fact that laser scanning is used by the entertainment industry to create highly believable yet unreal worlds is a little alarming.

"We've got to embrace these new techniques, but we must clearly understand that the digital world is an easy one to manipulate," warns Dr David McLay of the Association of Forensic Physicians. "Not only that, but we must ensure that there is no intentional or even unintentional falsification of the crime scene and what can be deduced from it."

He also believes it is too easy to be seduced by new technology. "From the point of view of jurisprudence, I would hope that there is always somebody there to challenge this and bring people down to earth a little bit."

The advocates of laser scanning agree that data, by its very nature, can be tampered with. But they insist that what is important in a criminal investigation is preserving the chain of evidence.

"When laser scanning data is captured, it is date-stamped and strict protocols are put in place to ensure integrity and continuity of evidence," says Tony Grissom of the California company Leica Geosystems, one of the leading manufacturers of laser scanning equipment. "While it is conceivable that data could be tampered with - intelligence agencies like MI5 or the CIA, for example, have the ability to do pretty much anything - it is an extremely unlikely scenario."

Although the main thrust of 3D laser scanning product development has been in the US, the technique is more commonly used by police in Britain. According to Raymond E Foster, a retired Los Angeles Police Department lieutenant and the author of Police Technology, the main reason for the slow take-up in the US is fragmentation. "Unlike the UK, which has 44 police agencies answering to the Home Office, we have more than 18,000 state and local police agencies answering to their city, county or state. Getting them on the same page when it comes to advances in technology is not easy," he says.

Looking back on his time in the Los Angeles force, Foster can recall many incidents where 3D laser scanning technology could have been used to good effect. "I can think of a ton of homicides where the ability to go back to the scene could have proved the key to cracking the case."

SEND IN THE SCANNERS

Three-dimensional laser scanning systems work by projecting a laser beam across an area. The laser light bounces off objects and returns to a digital sensor on the scanner. Laser scanning essentially measures millions of points at a scene - referred to as the "point cloud". These are converted into an appropriate digital format, and are used to recreate the area that has been scanned. Any desktop or laptop computer can take the laser-scanned data file and display the material as a 3D image that can be explored, much as you move around in a computer game.