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Digital evidence – friend or foe?

December 01 2010

Forensic Science Interpreting The Evidence

CCTV cameras monitor our activities in city centres, shops and on our roads, ostensibly to deter or detect crime. But such recordings are only a small fraction of the data stored as we go about our daily lives.  Everyday objects from mobile phones to sophisticated televisions incorporate computing capacity and memory. It is no surprise, therefore, that stored digital data from an increasing range of devices is being used in criminal cases, by prosecution and defence. 

There are many cases where incidental imagery of events has been key to their resolution. The infamous ‘cat in the wheelie bin’ received national news coverage and lead to a guilty plea. In other cases, imagery has exonerated defendants. One such example is where blood pattern evidence indicated a defendant had been involved in a street fight. Our examination of imagery subsequently confirmed the defendant’s role as Good Samaritan attempting to separate individuals in the assault rather than as a protagonist.

The availability of camera functions on many devices together with facilities to share content on social networking sites has resulted in a substantial increase in digital photographs being available in the public domain. These have been used to demonstrate links or relationships between parties or events, including substantiating alibis. Forensic computing may be required to establish authenticity and when images were taken.  It may also be used to recover deleted images if they have not been fully overwritten. 

In criminal cases, forensic computing has for a long time focussed on the identification and classification of indecent images of children or investigation of fraud. As the use of computers has broadened and software has become more sophisticated, computer usage patterns are now investigated to see if it is possible to link activity with a particular user. An interesting result is that the defence alibi of ‘home alone’ can now be substantiated if the user has been logged-on. 

The providers of mobile telephone services maintain records of calls made and received for billing purposes.  These can be used to demonstrate communication between mobile telephones and thereby association of mobile telephone users. The information has been used to illustrate those involved jointly in, say, a series of crimes such as the theft of prestige cars or drugs supply. 

Equally, records of mobile telephone activity and information retrieved from a handset and SIM card is often helpful to the defence in cases where traditionally it has been one person’s word against another. In harassment and rape allegations, forensic examination of mobile phone equipment can provide a valuable insight into the nature and extent of pre and/or post incident communications, as indeed can other sources of similar information e.g. Facebook, bebo, Windows Live Messenger and e-mail accounts. 

Mobile telephone service providers also record which masts were utilised in making mobile telephone calls.  These can be used to locate and verify an area where calls were made and enable route tracing where a number of calls have been connected through a series of masts. Similarly, some consumer sat-navs also store route details when switched on and many commercial vehicles now have digital tracking systems which can be interrogated to provide time/place data, in addition to the more familiar driver behaviour evidence available from tachographs.

Digital data is available and can be used to provide answers to fundamental questions of who, what, where and when in criminal cases. Just as digital data can be part of the prosecution’s evidence, it can also be a valuable resource for the defence.

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