LIKE many computer addicts, Andrew Harvey and Jordan Bradley spent hours locked away in their bedrooms logging on to the Internet and tinkering with their PCs.

But while other users visited their favourite websites, shopped on-line or checked out the latest football scores, Harvey and Bradley had far more sinister intentions.

It was here, in a small corner of North-East England, hidden away from the rest of the world, that the cousins, working with a group of American men, created a devastating computer virus that ripped through computers worldwide with terrifying repercussions.

Part of an international hacking group called the Thr34t Krew, they developed a self-replicating worm that could attack computers with dire consequences.

Chris Metherell, special casework lawyer for Durham and Cleveland Crown Prosecution Service, said: "This was very technically sophisticated. It was a very clever piece of kit."

The tk worm - known as a Trojan because it hides in an innocent looking e-mail file - allowed the hackers to take over company networks by downloading files and wreaking havoc with confidential data.

It had a terrifying power capable of bringing major corporations to their knees, a strength that astounded even the most seasoned IT experts.

It was the most sophisticated virus ever investigated by the Metropolitan Police's National Hi-Tech Crime Unit.

The worm was so clever that it could take over control of a computer, access data and look for other machines to infect, then leave without virtually any trace of it being there.

It was able to access computers, work out how fast or hi-tech they were then leave if it didn't like the machine's capacity.

The worm was also programmed to exit a PC and then prevent other hackers from accessing the system and could command infected machines to disconnect other computers that were connected to the Internet.

There was also evidence that the Thr34t Krew used other people's computers to store illegal software.

Mr Metherell said: "When somebody wanted to buy something, they would go into that machine and download it.

"Because they have entire control over a machine they can look at the contents of the disk. If you are a company, they can look at your customer details, credit card information.

"They can literally access anything."

More than 19,000 computers across the world were affected. Even the US Army's computer network was infiltrated.

Luckily for them, the trojan was discovered before it had the chance to become widespread.

Had it hit with the ferocity of the Blaster virus that infected tens of millions of computers last year, the effect would have been incalculable.

As it was, the damage caused by the worm is estimated to be in the region of £5.5m not including the £300 cost per computer paid to remove the virus.

The US's Computer and Technology Crime Unit Hi-Tech Team (Catch) first started investigating the virus in December 2002 after discovering the worm on a computer in California.

Further investigation revealed the worm had originated in England and the trail, led by the National Hi-Tech Crime Unit, led officers to County Durham three months later.

Investigators identified Harvey and Bradley and in February 2003 they swooped on their homes in pre-dawn raids carried out by Durham police and the unit.

Three officers from Catch were also present at the raids after travelling from their base in southern California.

Computer equipment was seized from all the suspect's homes and was subjected to detailed examination leading the officers to charge Harvey and Bradley in September with committing offences under the Misuse of Computers Act 1990.

Officers discovered the tk worm had exploited a vulnerability in Microsoft's server software and using the group's name, it provided a backdoor to any machine that had it installed.

Seth Fogie, a US wireless security consultant, first became aware of the virus when a client called to say that his Internet connection was slow.

He discovered hackers had taken over the computer and had turned it into a server containing more than 3GB of illegal software.

After a lengthy investigation, Mr Fogie traced the virus to the Th34t Krew web page and even chatted with one of the hackers on-line, who told him he was 16-years-old and from England.

In an article detailing the discovery, called Close Encounters of the Hacker Kind, Mr Fogie said: "He told me all about his Krew, the measures of security they go through to maintain anonymity and the breadth and power of their network.

"In short, this 16-year-old guy and the other ten members of his Krew had created a worm that had more power and bandwidth than some governments."

What was apparent though, was that this was not the work of amateurs. Bradley and Harvey had a computer know-how that far exceeded the average user.

The pair were no doubt spurred on by just how far their IT knowledge could take them and they invited other hackers to join in the destruction.

But what started as a challenge on their home computers, resulted in devastating consequences.

Only this time, it is the hackers who are on the receiving end.

* Raymond Paul Steigerwalt, 21, of Indiana, US, was sentenced to 21 months in jail and ordered to pay $12,000 after pleading guilty to being involved in the Thr34T Krew and possessing child porn.