WASHINGTON — Loretta Lynch was a federal prosecutor in New York when she encountered an astonishing case of police brutality: the broomstick sodomy of a Haitian immigrant in a precinct bathroom. The 1997 assault on Abner Louima set off street protests, frayed race relations and led to one of the most important federal civil rights cases of the past two decades — with Lynch a key part of the team that prosecuted officers accused in the beating or of covering it up. President Barack Obama’s …