The federal government Thursday filed a class-action lawsuit that accuses New York City of shortchanging the pensions of current and former cops who served on active military duty after 9/11.

The lawsuit alleges that the city failed to apply federal law in calculating the pension benefits of police officers and improperly reduced their benefits, according to a prepared statement by Manhattan U.S. Attorney Preet Bharara.

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"The purpose of this lawsuit is to ensure that soldiers remain on the same footing as their civilian counterparts and receive all of the benefits to which they are entitled, and that they are not penalized for their service by the unlawful calculation of those benefits," said Bharara.

Legal papers didn't disclose the possible size of the class. But in a court appearance Monday for three individual cases originally filed on the issue, an assistant U.S. attorney said that there were close to 1,500 cops who took some kind of military leave from Sept. 11, 2001, to the present. A city lawyer estimated that as many as 1,000 current or retired cops might be affected.

"The law requires that employees who are called up for military duty be treated the same as any other employee who goes out on leave," city attorney Georgia Pestana said in a statement. "We believe that the pension benefits we provide to police officers who have served in the military meet that standard -- and would not want to discourage anyone from military service by providing them with less than what they are entitled to."

One of those who filed an individual lawsuit on the issue is Michael V. Doherty, who, according to court papers, lives in Lindenhurst. Court papers stated Doherty was a member of the Coast Guard reserve who served on active duty in 2003, as well as from 2004 to 2007. He retired as a third grade detective in January 2004, according to the complaint in the case.

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Doherty didn't return a telephone call for comment.