When Gary McKinnon was 14 years old he was given an Atari 400, one of the earliest home computers. McKinnon used it to play rudimentary computer games, create graphics and write programs, displaying a precocious talent that developed almost as fast as the new technology.

A few years later the young techie went to see the film WarGames, in which a nerd, not unlike himself, brings the world to the brink of war by hacking into the Pentagon computer network. McKinnon began to wonder how easy that would be. He didn’t want to start a war. But he could use his self-taught skills to search for proof of his other passion, UFOs.

It was a plan he began to put into action more than a decade later. In the film, Pentagon officials avert global conflict, forgive the young nerd and a promising future in computer science beckons. McKinnon, on the other hand, faces a bleak future and the prospect of spending the rest of his life in jail.

Last week he appeared before three appeal judges at the House of Lords to challenge an earlier ruling that he should be extradited to the United States to stand trial for what the authorities describe as “the biggest military computer hack of all time”.

McKinnon, from Glasgow, is accused of causing $700,000-worth of damage by hacking into 97 America military computers at the Pentagon and Nasa between February 2001 and March 2002. He is charged with stealing 950 passwords and deleting files at Earle naval weapons station in New Jersey. If he is found guilty, McKinnon faces up to 70 years in prison.

It’s a frightening prospect for a self-confessed geek who admits to the hacking offences, but claims he was motivated by curiosity, not malice: “I have always had an interest in UFOs and science fiction,” he says. “Finding evidence of UFOs was a fantasy I’d toyed with and I thought I could explore that through hacking.”

McKinnon, quietly spoken with a dry sense of humour, doesn’t come across as a fantasist, but he is more convinced of extraterrestrial activity now than he was before he hacked into the American military network.

Some have suggested he is exaggerating his interest in UFOs to sound mad so that he will be judged unfit to stand trial. He says that’s not true, stressing he has always been upfront about what he was looking for. “I was a one-issue person,” he says. “I didn’t have any ulterior motive.”

McKinnon can’t claim to have found proof of aliens — although he tries — but he did discover just how simple it was to access highly sensitive information.

“It was ridiculously easy,” he says. “I am not some clever criminal mastermind who worked out a strategy. I went on a fishing expedition for blank administrative passwords — ones that had never been changed — and was incredibly surprised by how many I found even at high levels. There were about 5,000 on computers across the military network.”

Once he had hacked into the network, McKinnon could access different computer systems with ease as it appeared he was a trusted user, operating from within. In fact McKinnon was more than 3,000 miles away in a bedroom of his girlfriend’s aunt’s house in north London feeling increasingly cocky about his ability to snoop without getting caught.

When he started out, McKinnon, now 44, was working as a systems administrator. His hacking was confined to a few hours every evening with longer spells at weekends. Gradually his hobby developed into a full-blown addiction. He left his job and began spending eight hours a day sifting through military documents.

“I treated it a bit like a job,” he says. “I was pretty organised and very determined. I thought what I was doing was worth something.”

As he grew more confident McKinnon also became careless. He would drink a few beers while hacking and spend longer periods of time poking around sites where his unauthorised activity could be detected.

The day he got caught, McKinnon was downloading a grainy black-and-white photograph he believes was an alien spacecraft. He found it on a Nasa computer housed in the Johnson Space Center in Houston, Texas, and describes it as “very striking, not man-made”. Convinced he had found real evidence, McKinnon began to download the photograph to his computer via his dial-up connection. Before the download was complete McKinnon says he was disconnected. Somebody had noticed his presence and closed him down.

For the authorities, tracking down McKinnon proved straightforward. He had used his own e-mail address to download a remote-access program, which led the police to his door. In November 2002 he was awoken at 7.30am by officers from the National High Tech Crime Unit, who took him in for questioning.

From the beginning McKinnon admitted to hacking, but tried to blame the extent of his activity on lax security. If his cyber-fingerprints were all over the American military computer network, it was because nobody had stopped him. The American authorities offered a plea bargain. If he voluntarily travelled to the US to stand trial, he would get a sentence of no more than three or four years. McKinnon turned it down, claiming he should be tried in Britain, where the offences originated.

His plea to be tried in Britain, however, was rejected by John Reid, the former home secretary, who agreed his extradition under a 2003 treaty between Britain and the US that was designed to speed up the transfer of terrorist suspects. McKinnon, who is not accused of terrorism offences, believes he has been trapped in that net. He has voiced fears about being sent to Guantanamo Bay and tried by a military tribunal, although a judge at an earlier hearing described this concern as “fanciful”. If extradited, McKinnon will probably face trial in Virginia, prosecuted by the computer crime and intellectual property section of the Department of Justice.

It is now almost six years since McKinnon’s arrest. As part of his bail conditions he is not allowed to use e-mail or any computer connected to the internet. To fill the time, he works occasionally as a fork-lift truck driver, although he says the money isn’t as good as the computer business.

The House of Lords’ appeal decision is expected in three to four weeks. If they don’t overturn the earlier ruling, McKinnon will appeal to the European Court of Human Rights, but he could still be put on a plane to the US ahead of any hearing in Strasbourg.

“I am a pretty strong person. If that happens I will take my fight to America,” he says. “It’s a scary thought, though. I hope it doesn’t come to that.”

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