Immigrants caught one of L.A.’s most notorious serial killers. Lessons from the barrio 40 years later.

On a toasty summer Sunday 40 years ago, Richard Ramirez was the nation’s most wanted man. The Nightstalker serial killer had spent months on a reign of terror, casually slipping into homes across Southern California and killing a total of 15 people.He had eluded the police all summer, but they finally identified him and released his photo to the public. He emerged that Sunday morning on a bus at the edge of downtown and moved east into Boyle Heights, the heart of Latino L.A. and the entry point for countless undocumented immigrants coming from Latin America in the 1980s. What...