The 122-page agreement, known as a consent decree, states that “the ability of a police department to protect the community it serves is only as strong as the relationship it has with that community.” It calls for hundreds of new department policies governing the use of force, searches and seizures, arrests, interrogations, photographic lineups and more.

The agreement also deals with recruitment and training, performance evaluations and promotions, misconduct-complaint issues and even the lucrative off-duty work assignments that had become a potent source of corruption.

Over the years, the Justice Department has signed many such consent decrees with local law enforcement agencies over excesses in a single area, like an overuse of serious force or racial profiling. But the New Orleans agreement is notable for the broad array of issues it addresses; Mr. Holder called the agreement a “critical step forward” that could “serve as a reference point — and a potential model for success — in our future efforts.”

The agreement, which is subject to approval by a federal judge, has been long in coming. Days after taking office in 2010, Mayor Mitch Landrieu announced that he was inviting the Justice Department to help clean up a law enforcement agency that had grown increasingly lawless, saying, “We have a systemic failure.”

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