It happened just after 5 p.m. at the ex-girlfriend's home in Bensonhurst, where police say 31-year-old Officer Yvonne Wu was waiting for the couple to arrive.
Wu allegedly opened fire, wounding her 23-year-old ex-girlfriend and killing that woman's new partner, 24-year-old Jamie Liang.
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Police believe the incident was premeditated and that Wu used her service weapon.
The 23-year-old was shot in the torso but managed to call 911. She is expected to survive.
Liang was shot in the chest and was pronounced dead at the scene.
Wu, a 5-year veteran of the force assigned to the 72nd Precinct covering Park Slope and Sunset Park, waited outside for officers to arrive and told them she had shot two people.
"We believe it is domestic in nature," NYPD Assistant Chief Michael Kemper said. "We believe they had an intimate relationship."
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Wu was taken into custody and charged with murder and attempted murder.
"The whole incident is horrible, but these cops performed great, just heroically, and this is what NYPD cops come upon every single day," Kemper said. "Is this an incident they would want to come upon? No. But unfortunately throughout their careers they come upon this."
Wu worked the midnight tour with a clean record and no obvious disciplinary issues in her past.
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