(AP) -- New York City Police Commissioner William Bratton is leaving the nation's largest police force, after a tenure in which he received credit for keeping crime down and navigated tension between police and minority communities.

Mayor Bill de Blasio announced Tuesday that Bratton will retire next month to enter the private sector, although he and Bratton wouldn't disclose details. James O'Neill, the department's top chief, will become commissioner.

Bratton, who led the department in the 1990s before returning in 2014, noted that he was leaving at "a challenging time for police in America and New York, even though all indicators are pointing in the right direction."

He said no department is better prepared to confront "the crises of race in America, crime in America, the threat of terrorism" and the divisiveness of the presidential election.

De Blasio called Bratton's contributions to the city "inestimable and extraordinary," while heralding O'Neill as someone who would "take this department where it's never been before" by leading a push toward neighborhood policing, or trying to build trust and working relationships between law enforcement and communities. O'Neill has been heavily involved in the city's plans to shift toward that strategy; de Blasio said neighborhood policing would be in place in 51 precincts as of this fall.

"We've tried to redefine our relationship from being the police to being your police," Bratton said.

Yet New York Police Department critics said Bratton hadn't come close to ending discriminatory and abusive policing, and they questioned whether elevating his second-in-command would make a difference.

"So-called 'community policing,' 'training' and the rhetoric of 'police-community relations' are no solution to the systemic problems with policing in this city and nation," Communities United for Police Reform, a group that advocates for changing police practices, said in a statement.

O'Neill, a Brooklyn native, started as a patrolman in the transit system over 30 years ago. He has been chief of department, the department's highest uniformed position, since November 2014.

Bratton's resume is unmatched in local law enforcement. He began his career as a patrolman in Boston in 1970 and went on to lead law enforcement agencies in Boston, Los Angeles and New York.

In his first tenure at the NYPD under Mayor Rudolph Giuliani in the early 1990s, he was credited with driving down crime with a widely copied, data-driven, crime-fighting strategy before his brash style made him an annoyance to the mayor, who forced him out.

De Blasio was elected as a sharp critic of the stop-and-frisk tactic, which involved stopping and searching huge numbers of young black men. He picked Bratton as a sign that he would balance reform with further efforts to drive down crime.

On Bratton's watch, the NYPD has drastically scaled back stop-and-frisk, but stepped up enforcement against of so-called "quality of life" offenses. Critics said that approach -- known as "broken windows," shorthand for the theory that reining in small crimes helps deter more serious ones -- unfairly targeted minorities and came into play in the chokehold death of Eric Garner during his arrest for allegedly selling loose cigarettes. Garner, who was black, was unarmed; Officer Daniel Pantaleo, who put his arm around Garner's neck, is white.

Adding to a national wave of concern about police treatment of minorities, Garner's 2014 death and a grand jury's decision not to indict Pantaleo sparked protests and tension between the mayor and rank-and-file officers who felt he took protesters' side.

Then Officers Wenjian Liu and Rafael Ramos were ambushed and killed by a gunman who had announced online he planned to kill police in retaliation for Garner's death. In an extraordinary display of scorn, officers turned their back on the mayor at a hospital on the night of the December 2014 killings and again at the officers' funerals.

Bratton found himself in the middle, calling the officers' gesture inappropriate but at the same time noting that it reflected their feelings about "many issues."

The tensions between the mayor and police eased, but officers have been under scrutiny this summer as concern about police-minority relations has welled anew here and elsewhere.

Still, Bratton said, "we are farther along in New York City than most places" in improving relationships between police and minorities, thanks to training and other measures.

Meanwhile, the department has been facing an ongoing corruption probe that has spurred charges against two high-ranking officers and a businessman. Prosecutors say the officers accepted $100,000 in free flights, prostitutes, meals and other bribes, and in return arranged for police escorts, special parking and gun permits.

As tensions between the police and minorities have grown, the mayor, were he to be re-elected next year, will likely be under pressure by his liberal allies to select a more progressive candidate, and likely a commissioner of color.

O'Neill, like Bratton and de Blasio, is white.

___

Associated Press writer Jonathan Lemire contributed to this report.

___

This story has been corrected to show that Garner and officers Liu and Ramos died in 2014, not 2012.