An off-duty police officer allegedly shot her ex-girlfriend and killed the woman's new partner after catching the pair together, authorities have said.

Yvonne Wu, 31, reportedly opened fire on the couple at the ex-girlfriend's house in Bensonhurst, New York.

The pained lover was reportedly waiting outside of the home when police arrived and explained in an extremely "calm and collected" manner that she had shot "someone" inside the house, cops said in a press briefing.

Rocco Perna, a 44-year-old neighbour, said: "She didn’t say anything. 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."

When cops ventured inside, officers found Wu's ex-girlfriend's new lover Jamie Liang, 24, laying on the living floor with at least one gunshot wound to her chest, NYPD Assistant Chief Michael Kemper said.

The 23-year-old ex-girlfriend was also found lying on a bedroom floor with a bullet wound in her torso, New York Post reports.

Both injured women were rushed to a nearby hospital, where Liang died, police reported.

Wu's ex-lover is currently in a stable condition.

Kemper said: "This horrific incident is being treated as a homicide case."

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Investigators believe that Wu was already at the home before the two victims arrived back, Kemper says.

Detectives are looking to see if the woman trespassed onto the property.

A 911 call made from the home during the shooting reportedly painted a haunting picture of the bloody ordeal as it unfolded, according to a police source.

In the emergency call, someone can be heard saying: "I told you not to mess with me," the source said.

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Another woman can allegedly be heard shouting: “No Please. No Please.”

Four loud "bangs" can then reportedly be heard on the audio, according to the source.

It has not been confirmed if Kemper used her service weapon in the shooting.

The off-duty police officer reportedly dated her ex for two years before breaking up around three years ago.

Wu has received 4 medals during her service in the police force and always had good evaluations and feedback, a source said.

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