how he was caught

"He was caught after US military agencies detected system intrusions which were traced back to the UK"

According to Gary ..

"I'd instant message them, using WordPad, with a bit of a political diatribe. You know, I'd leave a message on their desktop that read 'Secret government is blah blah blah.' Gary McKinnon

And the alleged 'hacking` took the form of logging into Windows with no admin password as they were all installed from the same image.