The first trip spawned from an exposÃ Thompson was writing for Rolling Stone magazine about the Mexican-American television journalist Ruben Salazar , whom officers of the Los Angeles County Sheriff 's Office had shot and killed with a tear gas grenade fired at close range during the National Chicano Moratorium March against the Vietnam War in 1970 . The first trip spawned from an exposÃ Thompson was writing for Rolling Stone magazine about the Mexican-American television journalist RubÃ n Salazar , whom officers of the Los Angeles County Sheriff 's Department had shot and killed with a tear gas grenade fired at close range during the National Chicano Moratorium March against the Vietnam War in 1970 .