By Clark Boyd Technology correspondent

The US alleges Mr McKinnon attacked sites soon after 9/11

To hear the US government tell it, Gary McKinnon is a dangerous man, and should be extradited back to America to stand trial in a Virginia courtroom.

One US prosecutor has accused him of committing "the biggest military computer hack of all time".

If extradited, Mr McKinnon could face decades in US jail, and fines of close to $2m.

'Bumbling nerd'

The charges against Mr McKinnon are extensive.

The US government alleges that between February 2001 and March 2002, the 40-year-old computer enthusiast from North London hacked into dozens of US Army, Navy, Air Force, and Department of Defense computers, as well as 16 Nasa computers.

It says his hacking caused some $700,000 dollars worth of damage to government systems.

What's more, they allege that Mr McKinnon altered and deleted files at a US Naval Air Station not long after the terrorist attacks on September 11, 2001 and that the attack rendered critical systems inoperable.

The US government also says Mr McKinnon once took down an entire network of 2,000 US Army computers. His goal, they claim, was to access classified information.

In July 2005, Mark Summers, another official representing the US government, told a London court that Mr McKinnon's hacking was "intentional and calculated to influence and affect the US government by intimidation and coercion".

'Hacker' extradition case reopens

But Gary McKinnon, or Solo as he was known online, paints a very different picture of himself, and his motivation. In a BBC interview last summer, Mr McKinnon said that he was not a malicious hacker bent on bringing down US military systems, but rather more of a "bumbling computer nerd".

He said he's no web vandal, or virus writer, and that he never acted with malicious intent.

But he did admit that he hacked into dozens of US government computer systems. In fact, he calmly detailed just how easy it was to access extremely sensitive information in those systems.

"I found out that the US military use Windows," said Mr McKinnon in that BBC interview. "And having realised this, I assumed it would probably be an easy hack if they hadn't secured it properly."

Using commercially available software, Mr McKinnon probed dozens of US military and government networks. He found many machines without adequate password or firewall protection. So, he simply hacked into them.

UFO search

But for some, his method of hacking is not nearly so interesting as his reason for doing it.

Mr McKinnon got his first computer when he was 14 years old, and has been a hobbyist ever since. He left school at 17, and became a hairdresser. But, in the early 1990s, some friends convinced him to get a qualification in computers. After completing a course, he started doing contract work in the computing field.

By the late 1990s, Mr McKinnon decided to use his hacking skills to do what he calls "research" on an issue he firmly believes in. Mr McKinnon told the BBC that he is convinced that the United States government is withholding critical information about Unidentified Flying Objects.

"It wasn't just an interest in little green men and flying saucers," said Mr McKinnon. "I believe that there are spacecraft, or there have been craft, flying around that the public doesn't know about."

Mr McKinnon further explained that he believes the US military has reverse engineered an anti-gravity propulsion system from recovered alien spacecraft, and that this propulsion system is being kept a secret.

The US alleges that Mr McKinnon attack the base at Fort Meyer

In that sense, Mr McKinnon said he sees his own hacking as "humanitarian." He said he only wanted to find evidence of a UFO cover-up and expose it. He called the alleged anti-gravity propulsion system "extra-terrestrial technology we should have access to".

"I wanted to find out why this is being kept a secret when it could be put to good use," he said in the BBC interview last year.

Gary McKinnon's search turned into an obsession, an addiction. As he probed high-level computer systems in the United States, his life in Britain fell apart. He lost his job, and his girlfriend dumped him. Friends told him to stop hacking, but to no avail.

"I'd stopped washing at one point. I wasn't looking after myself. I wasn't eating properly. I was sitting around the house in my dressing gown, doing this all night."

Net lockdown

Eventually, Mr McKinnon got sloppy. He started leaving behind clues. At one point, Mr McKinnon began posting anti-war diatribes on the screens of the US government computers that were his targets. He has insisted, however, that he never attempted to sabotage any operations.

When Britain's hi-tech crime unit finally came for him 2002, Mr McKinnon was not surprised. He told the BBC: "I think I almost wanted to be caught, because it was ruining me. I had this classic thing of wanting to be caught so there would be an end to it."

He thought he would be tried in Britain, and that he might get, at the most, three to four years in prison.

The US military's use of Windows let Mr McKinnon in

Then, later that year, the United States decided to indict him with charges that could mean up to 70 years in a US prison. It has never been entirely clear why it took US officials until 2005 to begin extradition proceedings.

Gary McKinnon's been fighting extradition ever since, on the grounds that he never intended anything malicious by his hacking. He's been free on bail, but it has been a strange kind of freedom.

Until recently, he had to sign in at his local police station every evening, and could not leave his house at night. The court also forbade him from using any computer connected to the internet.

Some of those restrictions were eased this past Christmas. He can now use the internet, but the authorities are making him register the IP address with the local service provider.

Mr McKinnon remains contrite about what he did, although he has admitted that he thinks US officials are making him a scapegoat. He has said that in the course of his hacking, he found evidence that hundreds of others from around the world were also trying to hack the same networks.

His supporters say that instead of prosecuting him, the US government should thank him for pointing out massive computer security lapses in critical systems.

As for his quest to find evidence of a UFO cover-up, Mr McKinnon has said that he found some circumstantial evidence online to back his claims, including what he said are photos with what he speculated were alien spacecraft airbrushed out of the picture.

He said the photos in question were too large to download to his own computer.

When the BBC asked him last summer if he ever felt like hacking again, Mr McKinnon replied, "No, not at all." He said he wished he had listened to his friends when they told him, nearly three and a half years ago, to stop.

Clark Boyd is technology correspondent for The World, a BBC World Service and WGBH-Boston co-production.

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