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New York City Police Commissioner James O'Neill resigned Monday after three years as the head of the nation's largest police force, reports WABC.

"I came into this job with one mission: to fight crime and keep everybody safe," O'Neill said. "We did it and continue to do it."

Mayor Bill de Blasio held an afternoon news conference to officially make the announcement, thanking O'Neill for his service and naming Chief of Detectives Dermot Shea as his replacement.

The mayor met with senior police officials over the weekend, including extensive meetings with Shea and First Deputy Commissioner Ben Tucker. He called Shea, who will take office on December 1, a "proven change agent."

"This is a tremendous honor and a tremendous responsibility and I'm grateful to the mayor for this privilege to serve," Shea said. "Police Commissioner O'Neill has been a mentor and a friend to me, and I am committed to building on the incredible success of Neighborhood Policing and precision policing, while continuing my life's work to eradicate gangs and guns from our streets. Every New Yorker deserves to be safe and feel safe, and that has been my mission since I took the oath and became a police officer 28 years ago. As police commissioner, this will be what drives me."