State investigators have searched the home of a Los Angeles Police Department officer they believe played a key role in mailing out thousands of racially charged letters to Latino voters in an Orange County congressional district this month. The officer has been identified as Mark Nhan Nguyen, 32, a three-year veteran of the force who works as a collision investigator in South Los Angeles, according to sources familiar with an ongoing investigation by the state attorney general into the letter and whether the mailing violated any laws.