An off-duty NYPD cop shot her ex-girlfriend and killed the woman’s new lover after catching the duo together at a Brooklyn home on Wednesday night, police and sources said. Yvonne Wu, 31 years old and a five-year veteran of the force, opened fire on the pair at the ex-girlfriend’s house on 19th Avenue near 79th Street in Bensonhurst at about 5 p.m., according to police and law enforcement sources. The jilted lover was waiting outside the home when cops arrived and admitted in a very “calm and collected” manner that she shot “someone” inside the house, NYPD Assistant Chief Michael Kemper said at a Wednesday night press briefing. Yvonne Wu, 31, is accused of shooting her ex-girlfriend and another woman. “She didn’t say anything,” said neighbor Rocco Perna, 44. “She was so quiet. No emotions, nothing. She was handcuffed and they put her in the car, that was it. She didn’t fight back or nothing. Didn’t say a word.” Inside, the officers found Wu’s ex-girlfriend’s new 24-year-old lover, Jamie Liang, splayed out on the living room floor with at least one gunshot to her chest, Kemper said. The ex-girlfriend, 23, was found lying on a bedroom floor with a bullet wound to her torso. Both women were taken to Maimonides Medical Center, where Liang died, police said. Wu’s ex was listed in serious but stable condition, according to cops. The officer was in police custody Wednesday night at a local hospital, where she was being evaluated. “This horrific incident is being treated as a homicide case,” Kemper said. Investigators believe Wu was already at the home before the two victims arrived, according to Kemper. It wasn’t immediately clear how Wu entered the home, and detectives are looking to see if she trespassed onto the property. A 911 call was made from the address of the shooting that painted a dramatic picture of the chaos as it unfolded, according to a high-ranking police source.