NEW YORK New York City Police Commissioner Keechant Sewell, the first woman to lead the nation's largest police department, on Monday said she is resigning after serving 18 months in the post.

She gave no reason for stepping down.

The surprise departure was announced by Sewell, 51, to her colleagues in a brief letter, posted online by local television station WABC-TV, in which she praised the "compassion, heroics and selflessness" of members of the New York Police Department (NYPD).

"You are an extraordinary collective of hard working public servants dedicated to the safety of this city," she wrote. "You and your predecessors are the reason that the NYPD is known as the gold standard of law enforcement."

Mayor Eric Adams, himself a former New York police captain, appointed Sewell as the city's 45th police commissioner when he took office in January 2022.

Sewell, a 23-year veteran of the police department in Nassau County, New York, where she was chief of detectives, became the first woman and third African American, to head the NYPD, overseeing some 35,000 uniformed offices and 18,000 civilian employees.

Raised in the New York borough of Queens, Sewell succeeded Dermot Shea, who was appointed the city's top cop in 2019 by then-Mayor Bill de Blasio.

Adams credited Sewell with playing "a leading role in this administration's tireless work to make New York City safer."

"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," he said on Twitter.

Neither Sewell nor the mayor made clear the effective date of her resignation or made mention of a replacement.

The president of the Police Benevolent Association union, Patrick Lynch, also praised Sewell, saying in a statement that she made a "real impact" after "taking over a police department in crisis."

"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. "Her leadership will be sorely missed."

(Reporting by Jonathan Allen in New York; Writing and additional reporting by Steve Gorman in Los Angeles; Editing by Jamie Freed)