I think it probably will be tracked back to a "Republican" (who was a democrat 2 years ago).
There is more detail in the LA Times and OC Register versions of the story but they were too hard to excerpt.
http://www.latimes.com/news/local/la-me-letter19oct19,0,6413307.story?coll=la-story-footer
http://www.ocregister.com/ocregister/news/state/article_1324403.php
The articles don't mention what mis-statements were made in the letter, nor does it mention what laws were broken. Surely it cannot be illegal to tell people that it is illegal for non-citizens to vote. My guess is that the only thing illegal about the letter is the letterhead.