How did Klaus Fuchs get caught?

Fuchs in January 1950 confessed that he was a spy. Fuchs told interrogators the KGB acquired an agent in Berkeley, California, who informed the Soviet Union about electromagnetic separation research of uranium-235 in 1942 or earlier. He was prosecuted by Sir Hartley Shawcross and was convicted on 1 March 1950. He was sentenced the next day to fourteen years in prison, the maximum possible for passing military secrets to a friendly nation.