The first woman to lead nation's largest police department, resigned

After serving 18 months in the post..

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 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."

Sewell became the first woman and third African American, to head the NYPD, overseeing some 35,000 uniformed offices and 18,000 civilian employees.