In the criminal justice system, the people of Manhattan have been represented for 35 years by New York County District Attorney Robert Morgenthau. This is his story.Mr. Morgenthau, who inspired the original D.A. character on the television program “Law and Order,” will retire on Thursday at age 90. Much of the barely fictitious drama is set in his office in Manhattan’s Criminal Courts Building. This week, amid half-filled boxes and scattered personal mementos, he sat down to discuss his life’s work.Even though he knows I’m wearing a wire—actually an audio recorder placed on the table between us—America’s D.A. speaks candidly, including about his public blowups with New York Mayor Michael Bloomberg. Mr. Morgenthau says this is the first mayor he hasn’t gotten along with, and that the relationship went south when his office started investigating the city’s role in the death of two of New York’s bravest in an August, 2007 fire. Among other mistakes, city inspectors had failed to note that the water had been turned off at the old Deutsche Bank building opposite Ground Zero. The blaze resulted in 33 “mayday” calls from firefighters, and the D.A. is amazed that only two lost their lives.