NYPD Commissioner Keechant Sewell announced Monday she was stepping down after a year and a half on the job, Fox News has learned from law enforcement sources. 

"I have made the decision to step down from my position," Sewell said in a memo to the force. "While my time here will come to a close, I will never step away from my advocacy and support for the NYPD, and I will always be a champion for the people of New York City."

NYPD commissioner Keechant Sewell

Keechant Sewell, police commissioner of the New York City Police Department (NYPD), speaks during a news conference in New York, US, on Wednesday, Aug. 3, 2022.  (Photographer: Michael Nagle/Bloomberg via Getty Images)

Sewell, who was appointed by NYC Mayor Eric Adams, became commissioner of the nation’s largest police force after serving as chief of detectives in the Nassau County Police Department. 

The reason for Sewell’s departure was not given in the memo. 

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Mayor Adams thanks Sewell's "devotion" and her "steadfast leadership" over the last 18 months. 

"Her efforts played a leading role in this administration’s tireless work to make New York City safer. When we came into office, crime was trending upwards, and thanks to the brave men and women of the NYPD, most of the major crime categories are now down. 

Keechant Sewell at podium with Adams by her side

FILE: NYPD Commissioner Keechant Sewell speaks from the podium during press conference Wednesday morning at City Hall.  (Luiz C. Ribeiro for NY Daily News via Getty Images)

"The commissioner worked nearly 24 hours a day, seven days a week for a year and a half, and we are all grateful for her service. New Yorkers owe her a debt of gratitude."

Patrick J. Lynch, president of the Police Benevolent Association, said Sewell "made a real impact" in her short time with the NYPD. 

"She took over a police department in crisis and faced tremendous challenges from day one. She cared about the cops on the street and was always open to working with us to improve their lives and working conditions," Lynch said. 

"There are still enormous challenges facing the NYPD. Her leadership will be sorely missed."

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Sewell took over as commissioner when Adams became mayor in January 2022, having pledged beforehand to name a woman to the post.

Adams, a former police captain, took an outsized interest in the department's operations compared with other mayors. Since Sewell's appointment, Adams has faced questions about whether she has been granted the autonomy of past commissioners.

Almost immediately after taking office, Adams hired former NYPD Chief of Department Phillip Banks as his deputy mayor for public safety, somewhat diminishing Sewell's authority. Banks has been holding weekly public briefings on crime, often without Sewell in attendance.

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Sewell started with the Nassau County Police Department as a patrol officer in 1997, then became a precinct commander, head of major cases, a top hostage negotiator and finally chief of detectives, where she oversaw a staff of about 350 — about 1% the size of the NYPD's unformed ranks.

The Associated Press contributed to this rpeort.