Here you go, straight from the horse's mouth as you requested:

http://www.epa.gov/Compliance/resour...le/admem1a.pdf

This particular pdf is specific to alternative fuel conversions but states, in the first few paragraphs, where one can find the exact wording in regards to tampering et al, which if you don't care to read the entire pdf is Section 203(a)(3) of the Clean Air Act.

There are many addendums, memorandums, etc to that act which are all available at the U.S. Environmental Protection Agency website.

Hope this helps,

Dan