LONDON (Mar 21, 1997 8:25 p.m. EST) - A masterspy believed by the Pentagon to be the No. 1 threat to U.S. security and deadlier than the KGB turned out to be a British schoolboy hacker working out of his bedroom. U.S. military chiefs feared that an East European spy ring had gained access to their innermost intelligence secrets and hacked into American Air Defense systems. But a 13-month investigation and a dramatic police raid on his London home revealed that 16-year-old music student Richard Pryce was the culprit. Pryce, known on the internet as "The Datastream Cowboy," was fined $1,915 Friday by a London court after what his lawyer called "a schoolboy prank" reminiscent of the movie "War Games." The U.S. Senate armed services committee was told the mystery hacker was the number one threat to U.S. security. He was said to have downloaded dozens of secret files, including details of the research and development of ballistic missiles. Up to 200 security breaches were logged. Using a $1,200 computer and modem, Pryce hacked into computers at Griffiss Air Base in New York and a network in California run by the missile and aircraft manufacturer Lockheed. "Those places were a lot easier to get into than university computers in England," Pryce told reporters. "It was more of a challenge really, going somewhere I wasn't meant to. If you set out to go somewhere and you get there, other hackers would be impressed," he said. His prank put Pryce on the front pages of most British newspapers Saturday with tales of "The Schoolboy masterspy" and "The Boy who cracked open the Pentagon." Pryce, now 19, has been offered sizeable sums for the book and film rights to his story but his parents say he prefers to stick to his double bass and concentrate on winning a place in a leading London orchestra. "Quite remarkably in a society dominated by sleaze, he has refused all the offers and wants to resume his quiet life," said his father, Nick Robertson. His computer skills were not reflected in his exam results -- he was only awarded a 'D' grade.