The historical ties between American law-enforcement agencies and white supremacist groups are well documented. Southern sheriffs abetted Ku Klux Klansmen under Jim Crow. The FBI posted a bulletin in 2006 warning about white nationalists and skinheads infiltrating police entities, citing cases in Ohio, Illinois, Texas, and California, including the formation of a neo-Nazi gang by officials within the Los Angeles County Sheriff’s Department. The armed forces have been implicated as well. Federal agents in February arrested a 49-year-old Coast Guard lieutenant and self-described white nationalist who had amassed 15 firearms and more than 1,000 rounds of ammunition while planning a massacre of innocent civilians “on a scale rarely seen in this country,” according to court documents.