EL PASO — There’s a reason Ron Stallworth remembers everything in detail: the hate in their voices. Their constant use of racial slurs. The way they described, with heated anticipation, plans to terrorize black and Jewish people in Colorado Springs with cross burnings, marches and threats of violence, even as they sought to put a fresh, mainstream face on their racism. As the first black detective in Colorado Springs history, Stallworth went undercover with the Ku Klux Klan from October 1978 to April 1979, improbably duping both the local chapter and its young, fast-rising national leader, David Duke, into thinking Stallworth was a racist white man. Stallworth played the voice in phone conversations with Klan members while his partner showed up in person to Klan meetings in basements and churches around Colorado Springs.