NEW YORK — A New York City police officer serving in the Army Reserve was shot and killed by a sniper while guarding prisoners at a camp in Iraq, city officials said Wednesday.

Staff Sgt. James McNaughton, 27, is the first member of the force to be killed in action in Iraq, the police department said. He died Tuesday at Camp Victory, outside Baghdad.

McNaughton joined the New York Police Department in July 2001 and was assigned to its transit bureau, which patrols city subways.

Police work was the family business. His father is a retired New York police officer. His stepmother is an officer in the transit bureau. He was engaged to be married to an officer in the 9th Precinct.

McNaughton, whose family lived in Centereach, on Long Island, deployed to Iraq with the 306th Military Police Battalion, 77th U.S. Army Regional Readiness Command, based at Fort Totten, in Queens.

Mayor Michael Bloomberg said in a written statement announcing the death, “James McNaughton made our city safe as a police officer and gave his life defending our country.”

Police Commissioner Raymond Kelly said McNaughton “embodied the motto of the NYPD: Fidelis ad Mortem, faithful until death.”

McNaughton is one of 273 members of the police department on active duty.

A city firefighter, Christian Engeldrum, was killed in Baghdad in 2004 while serving with the Army National Guard.

Police Officer Brian Kenny, who worked with McNaughton protecting lower Manhattan’s transit systems after the Sept. 11, 2001, terrorist attack, called him a “squared-away guy” who looked young for his age but displayed a rock-steady professionalism.

“He was a military person,” Kenny said. “If your assignment that night was to work with Jimmy, you couldn’t get a much better assignment. You knew you were going to have fun and you knew the job was going to get done right.”

McNaughton served one tour of duty in the United States after the Sept. 11 attacks, returned to the police department, then left for a second deployment overseas, Kenny said.

A state law passed this year will ensure that McNaughton’s family will receive a full pension and death benefits, as if he had died while wearing his police uniform.