A witness in computer crime trials has called for the release of individuals convicted of child pornography offences based entirely on material found on their hard drives.

Jason Coombes, who has been involved in computer crime trials since 1994, issued a statement calling on police to be more thorough in their investigations.

The expert contends that third-party control of a PC which is then used to store or view child pornography images could lead to the conviction of innocent people. 

"I have seen numerous cases in which the computer forensic evidence proves that a third-party intruder was in control of the suspect's computer," said Coombes.

"Every person convicted of an electronic crime against a child based only on evidence recovered from a hard drive that happened to be in their possession should be immediately released from whatever prison they are now being held.

"There is simply no way for law enforcement to know the difference between innocent and guilty persons based on hard drive data circumstantial evidence."

Coombes added that it was the responsibility of the police to use key-logging and internet wiretaps to prove guilt beyond reasonable doubt.

He also described it as "outrageous" that someone could be convicted of such a crime based solely on the contents of their hard drive.

Coombes highlighted the case of Commodore David White, who was accused of accessing a US child porn website and was relieved of his command despite there being no hard evidence against him. He was found dead in his swimming pool the next day.

Investigations into child pornography have led to 4,000 arrests, 1,600 people charged and 1,200 convictions. In the UK there have been 34 suicides by the accused.

Professor Neil Barrett, professor of computer science at Cranfield University, and himself an expert witness at computer crime trials, said: "I have heard this argument before, predominantly from defence witnesses seeking to poke holes in a case.

"To say that everything [in a child porn prosecution] is to do with a hard drive is unreasonable. We would never bring a case purely on the basis of what's on the computer."

Professor Barrett explained that suspects are interviewed and their stories checked before action is taken. If their computer is clean, for example if someone else was using their credit card number, no action is taken.

He added that the onus is always on the prosecution to prove that the accused directly accessed the images.