New York City's police commissioner is retiring after three years in charge of the nation's largest police department, Mayor Bill de Blasio said Monday.

James O'Neill, 61, will be replaced by Chief of Detectives Dermot Shea, the mayor said.

O'Neill's tenure as commissioner - which began with a pipe bomb blast on his first full day in office in September 2016 - came as the city continued to grapple with its place as a top terrorist target, as well as tensions between officers and the community.

He moved the department away from the controversial "broken windows" theory of law enforcement, which viewed low-level offenses as a gateway to bigger crimes, while presiding over continued drops in crime.

He led the department's response to a deadly truck attack in 2017 and brought closure to one of the NYPD's lowest moments this summer in firing an officer in the 2014 chokehold death of Eric Garner.

"On behalf of all New Yorkers, I want to express deep gratitude to Jimmy O'Neill for dedicating his entire career to keeping our city safe," de Blasio said in a statement. "Jimmy transformed the relationship between New Yorkers and police, and helped to make the Department the most sophisticated and advanced in the country."

De Blasio called Shea a "proven change agent" who has worked to build trust between police and communities and is "uniquely qualified" to serve as the city's next police commissioner.

Joining the NYPD as a transit officer in 1983, O'Neill spent more than three decades with the department before becoming commissioner.

As commissioner, he led efforts to bolster community policing and repair the department's relationship with minority communities that had complained about innocent black and Hispanic men being caught up in aggressive enforcement of minor crimes.

At times, it appeared O'Neill was caught between loyalty to his men and women in blue and the progressive policies embraced by his boss, de Blasio, and pushed by police reform advocates.

In one example, O'Neill said he wanted some changes to a state law that keeps police disciplinary records secret, so the department could share outcomes of cases with the public, but did not support a full repeal.

After O'Neill fired Officer Daniel Pantaleo for Garner's death, the city's largest police union responded by calling for his immediate resignation.

Asked in recent weeks about rumors of his retirement, he said he had the "best job in the world."

(Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)