Donald Twilley is an investigator with the Federal Bureau of Investigation's civil rights section, and the partner of Marcus Friedman. Twilley is of African-American ancestry.
In January of 2001, he worked with Terry McCaleb and Jaye Winston on an investigation into the murder of Edward Gunn. Twilley and Friedman became convinced that Harry Bosch was Gunn's killer and were assembling a case against him.