The prison sentence the US justice department is seeking - should Gary be successfully extradited - is up to 70 years. What Gary was hunting for, as he snooped around Nasa, and the Pentagon's network, was evidence of a UFO cover-up.
He sat in his girlfriend Tamsin's aunt's house in Crouch End, and he began to hack. He downloaded a program that searched for computers that used the Windows operating system, scanned addresses and pinpointed administrator user names that had no passwords. Basically, what Gary was looking for - and found time and again - were network administrators within high levels of the US government and military establishments who hadn't bothered to give themselves passwords. That's how he got in.
His Bufora friends "were living in cloud cuckoo land", he says. "All those conspiracy theorists seemed more concerned with believing it than proving it." He wanted evidence. He did a few trial runs, successfully hacking into Oxford University's network, for example, and he found the whole business "incredibly exciting. And then it got more exciting when I started going to places where I really shouldn't be".
"Like where?" I ask.
"The US Space Command," he says.
He sounds to me like a virtuoso hacker, although I am someone who can barely download RealPlayer. I nod blankly as he says things like, "You get on to easy networks, like Support and Logistics, in order to exploit the trust relationship that military departments have between each other, and once you get on to an easy thing, you find out what networks they trust and then you hop and hop and hop, and eventually you think, 'That looks a bit more secretive.' " When I ask if he is brilliant, he says no. He's just an ordinary self-taught techie. And, he says, he was never alone.
"Once you're on the network, you can do a command called NetStat - Network Status - and it lists all the connections to that machine. There were hackers from Denmark, Italy, Germany, Turkey, Thailand ..."
"All on at once?" I ask. "You could see hackers from all over the world, snooping around, without the spaceniks or the military realising?"
"What was the most exciting thing you saw?" I ask.
"I found a list of officers' names," he claims, "under the heading 'Non-Terrestrial Officers'."
"Non-Terrestrial Officers?" I say.
"Yeah, I looked it up," says Gary, "and it's nowhere. It doesn't mean little green men. What I think it means is not earth-based. I found a list of 'fleet-to-fleet transfers', and a list of ship names. I looked them up. They weren't US navy ships. What I saw made me believe they have some kind of spaceship, off-planet."
"The Americans have a secret spaceship?" I ask.
"That's what this trickle of evidence has led me to believe."
"Some kind of other Mir that nobody knows about?"
"I guess so," says Gary.
"What were the ship names?"
"I can't remember," says Gary. "I was smoking a lot of dope at the time. Not good for the intellect."
Yes, he was hacking in the immediate aftermath of September 11, but only because he wanted to see if there was a conspiracy afoot. "Why did the building fall like a controlled series of explosions? " he says. "I hate conspiracy theories, so I thought I'd find out for myself."
"And did you find a conspiracy?" I ask.
"No," he says.
He strenuously denies the justice department's charge that he caused the "US military district of Washington" to become "inoperable". Well, once, he admits, but only once, he inadvertently pressed the wrong button and may have deleted some government files.
"What did you do then?"
"I thought, 'Ooh, bloody hell,' " he says. "And that's when I stopped for a while. And then my friend told me about Darpa. And so I started again."
They found Gary in the end because he'd used his own email address to download a hacking program called Remotely Anywhere. "God knows why I used my real email address," he says. "I suppose it means I'm not a secretive, sophisticated, checking-myself-every-step-of-the-way type of hacker."
During a later hack, McKinnon left an electronic note comparing U.S. foreign policy to 'government- sponsored ter*rorism'.
He wrote: `It was not a mistake that there was a huge security standdown on September 11 last year. I am Solo. I will continue to disrupt at the highest level.'