(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.
Off-duty St. Louis police officer accidentally shot and killed by on-duty officer
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.
According to a police statement, a call for "officer in need of aid" was put out at 12:56 a.m., and the third officer told the dispatcher they were taking Alix to the hospital.
She was pronounced dead shortly after arrival, the statement said.
Police have released no additional details about what led to the shooting, nor why the on-duty officers were at the apartment.
St. Louis Circuit Attorney Kimberly M. Gardner said in a statement the Missouri State Highway Patrol would assist her office in conducting an "independent investigation regarding potential criminal conduct" in connection with the shooting.
The statement notes the circuit attorney's investigation is part of the protocol in a police shooting to ensure transparency.
"This is a tragic situation," Gardner said. "The family of officer Alix deserves a complete, objective and thorough investigation of this incident. My office is approaching this criminal investigation with the highest level of seriousness and urgency."
The police department's Force Investigation Unit also is investigating.
Alix had two years of experience on the force, the SLMPD said. She leaves behind a husband, her parents and a sister, among other family members.
"Officer Alix was an enthusiastic and energetic young woman with a bright future ahead of her," SLMPD Commissioner John Hayden said in the statement.
The St. Louis police union released a statement in which it recognized that the public wants to "understand what happened."
"So do we," the union said. "But for now, we wait; we wonder; and we weep."
In addition to Alix, at least 10 other police officers have died in 2019, per the National law Enforcement Officers Memorial Fund. Five of them died in firearms-related deaths, three in traffic accidents and two others of unspecified causes.