(CNN) An off-duty St. Louis police officer was killed early Thursday when another officer "mishandled" a gun and shot her, according to the St. Louis Metropolitan Police Department.

The incident is now under investigation by the St. Louis Circuit Attorney's Office.

Katlyn Alix, 24, was off-duty when she was accidentally shot by an on-duty officer at an apartment, police spokeswoman Officer Michelle Woodling told CNN. Another on-duty officer was also at the scene.

The two on-duty officers -- described only as 29-year-old white men -- met Alix at the apartment. Police have not said who lived there.

While they were sitting in the living room, one of the officers "mishandled a firearm" and shot Alix in the chest, Woodling said in an email.

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