An F.B.I. agent who sought to investigate the federal immigration officer who fatally shot a 37-year-old woman in Minneapolis this month has resigned from the bureau, according to two people familiar with the matter. The agent, Tracee Mergen, left her job as a supervisor in the F.B.I.’s Minneapolis field office after bureau leadership in Washington pressured her to discontinue a civil rights inquiry into the immigration officer, Jonathan Ross, according to one of the people. Such inquiries are a common investigative step in similar shootings.