Tan D. Nguyen denied knowing anything about the letter in an interview Thursday with The Associated Press but said he fired a campaign staffer who may have been responsible for it. Nguyen's attorney said his client had no intention of quitting.

State and federal officials were investigating the letter, which was written in Spanish and mailed to an estimated 14,000 Democratic voters in central Orange County. It warns, "You are advised that if your residence in this country is illegal or you are an immigrant, voting in a federal election is a crime that could result in jail time."