In the decade since Pakistan admitted that it had a nuclear weapons program, and that the man in charge of it, Abdul Qadeer Khan, was selling the technology on the black market, it's been revealed where all the high tech equipment was coming from. It turns out that Japanese firms were selling Pakistan the industrial equipment, often with the understanding that it was going to be used for something non-weapons related. This was often a flimsy excuse to make the sale, and no press releases were put out announcing the sales. But there was a paper trail, and now many of those documents, and witnesses, are telling the tale.
Japanese firms have gotten into trouble for this before. In the 1980s, Japanese firms provided the Soviet Union with hardware and software needed to manufacture quieter submarines. The equipment supplied had non-military uses, but the Japanese didn't bother to let the U.S. Navy know about the sale. If the U.S. had known, they would have raised hell with the Japanese government.
Also during the 1980s, it was German firms that supplied Iraq with equipment needed to manufacture pesticide, or nerve gas. Pesticide basically is nerve gas tweaked to work on insects. Make the right tweaks, and pesticide becomes nerve gas that works on mammals. The German equipment was used mainly to make nerve gas, for use against Iranian troops then trying to battle their way into Iraq.
While the U.S. makes a big deal out of keeping American manufactured nuclear and chemical weapons manufacturing equipment out of circulation, many firms in Europe and Japan can also supply this stuff. Japan and European nations also have laws that restrict exports of these goods, but not everyone enforces the rules as strictly as the United States. Moreover, even U.S. firms sometimes manage to knowingly, or unknowingly, let forbidden goods get shipped to the wrong people.
Passing laws to prevent nations from getting nuclear, chemical or biological weapons production equipment do not work all the time. Such laws slow down the proliferation of these weapons, but cannot stop it.