The new Bronx district attorney, Darcel D. Clark, oversees the prosecution of crime in a dozen police precincts and scores of neighborhoods.

But it is one particular trouble spot that is her focus: Rikers Island.

New York City’s main jail complex, which sprawls across the island in the East River, lies within her jurisdiction and is at the top of her priority list. “Right now, it’s the worst neighborhood I have,” Ms. Clark told an audience at a recent town-hall-style meeting on reforming Rikers, in Harlem.

Since taking office in January, Ms. Clark has created a prosecution bureau that will eventually have a total of 30 assistant district attorneys assigned to Rikers cases full time, about triple the current number.

In another change, many of those prosecutors will be based on the island, along with investigators and support staff, in a new office being set up in a trailer. The office, which will be open five days a week, will be the first permanent physical presence on the island for a district attorney in recent times.