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For years, city officials have worked to promote New York as the safest large city in America. Its transformation from urban war zone in the early 1990s to shining metropolitan destination has long been a point of pride for the Police Department. For two decades, murder rates have continued to plummet.

But in the first three months of this year, that trend took a concerning turn in the opposite direction, driven by a steep spike in murders in Brooklyn. As of March 24, the borough had recorded 28 homicides so far this year, compared with 17 in the same period last year, a 64 percent increase.

During the same period, police officials said the number of murders in the rest of the city fell slightly, from 43 to 41. Murders went up in significantly in Manhattan, but fell in the Bronx, Queens and Staten Island.

Crime rates rise and fall periodically and it is too early to tell if the increase in killings foretells a new crime wave that would challenge the sense of security that has become part of the city’s identity. Over time the increase may be offset by quieter periods, and trends that appear worrisome in March often level off by August.