NYPD officer Yvonne Wu is accused of shooting her ex-girlfriend and her ex’s new lover in their Brooklyn apartment on Wednesday.

Wu, 31, is in police custody after authorities said she broke into the Bensonhurst home while she was off duty and opened fire on the pair.

Both victims were taken to a nearby hospital, where the new partner, 24-year-old Jamie Liang, was pronounced dead. The 23-year-old ex-girlfriend, who authorities have not named, is being treated for a gunshot wound to the torso and is expected to survive.

Police said that Yvonne Wu, a five-year veteran of the force, was found waiting calmly outside the home and told responding officers what happened.

“She didn’t say anything,” a neighbor told the New York Post. “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.”

“I saw them all the time together,” another neighbor told CBS 2. “Just like anyone else, you can come home and find your lover or someone with someone else and you snap.”

Inside, authorities found Liang mortally wounded on the living room floor, with at least one gunshot wound to the chest. The ex-girlfriend was discovered on the floor in the bedroom.

Police believe that Wu was waiting in the apartment for the couple before they arrived home, and are investigating the possibility that the she broke into the apartment.

One NYPD source told the Post that the victims made a 911 call as the attack was taking place. In the emergency call audio, one woman can reportedly be heard shouting “I told you not to mess with me,” followed by another begging “No, please!” and then four gunshots, according to the police source.

“This horrific incident is being treated as a homicide case,” NYPD Assistant Chief Michael Kemper said in a Wednesday night press briefing. “We believe it is domestic in nature, we believe they had an intimate relationship.”

Officers recovered a gun at the scene, which police believe is Wu’s service weapon, according to a local ABC affiliate.

Another NYPD source told the Post that Wu had dated her ex-girlfriend for two years before the pair split up three weeks ago. One source told CBS 2 that Wu and the ex-girlfriend had remained in touch since the breakup, but that their conversations had been “confrontational.”

Yvonne Wu, who reportedly worked in the department’s 72nd precinct in Brooklyn, was a “model officer” according to a senior police official who spoke with NBC 4.

That police source told reporters that Wu was a beat cop who worked the graveyard shift and “exceeded expectations” in her annual performance reviews, making what happened Wednesday night “out of left field.” She was reportedly scheduled to work that night before her arrest at about 5 p.m.

Wu is expected to be charged with murder, and ABC 7 reports that police believe the incident was premeditated. She was reportedly evaluated at a local hospital Wednesday night after being taken into police custody. It’s not immediately clear whether she has contacted an attorney.

“The whole incident is horrible, but these cops performed great, just heroically, and this is what NYPD cops come upon every single day,” Kemper told reporters. “Is this an incident they would want to come upon? No. But unfortunately throughout their careers they come upon this.”

ABC 7 reported that the state Attorney General’s office is investigating the incident. According to a local CBS affiliate, the state-level authorities are handling the case because it is officially considered a police-involved killing.