Professional, singular. All it really takes is one person with an ideological axe to grind and a moderate amount of skill, and you have instructions that others can use to turn off-the-shelf components into weapons.

It's happened before. P.A. Luty wanted to make a point about British gun control laws, so he built a submachine gun out of ordinary hardware store parts and wrote a book ("Expedient Homemade Firearms") containing detailed instructions. He got jailed over making these weapons, of course, but the point stands.