To hear the US government tell it, Gary McKinnon is a dangerous man, and should be extradited back to America to stand trial. One US prosecutor has accused him of committing the biggest military computer hack of all time". Mr McKinnon could face decades in US jail, and fines of close to $2m. 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. Mr McKinnon told the BBC that he is convinced that the United States government is withholding critical information about Unidentified Flying Objects. "I believe that there are spacecraft, or there have been craft, flying around that the public doesn't know about." 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. He said he only wanted to find evidence of a UFO cover-up and expose it.