Nathaniel Hendren, 29, is accused of killing Katlyn Alix, 24, last Thursday at his home, authorities said.

A US police officer was charged with first degree manslaughter in the death of a colleague, according to the St. Louis judicial district attorney.

Nathaniel Hendren, 29, is accused of killing Katlyn Alix, 24, last Thursday at his home, authorities said. She was on leave when the shooting took place and Hendren was on duty, according to the statement of probable cause in support of the charges.

According to the statement, Alix's partner Hendren and Hendren were playing with firearms when Hendren made a revolver.

Hendren emptied the bullets from the revolver and handed one of the bullets inside the pistol, spun the barrel and fired the pistol in the opposite direction to Alix, according to the statement.

The weapon has not discharged. Alix took the gun and pulled the trigger while showing Hendren, but she did not fire, the statement said.

Hendren recovered the gun from Alix, pulled the trigger and shot this time, hitting Alix in the chest, the document said.

The third officer at the scene was not identified. According to the statement of probable cause, he advised Alix and Hendren not to play with guns and left the room. He came back after hearing the shot.

Hendren was charged with first degree manslaughter and armed criminal acts related to the death of a police co-worker. Brian Millikan, his lawyer, declined to comment on CNN.

In response to a request for comment, Jeff Roorda, Commercial Director of the St. Louis Police Officers Association, told CNN that the SLPOA "was entrusted to the accused officer's attorney".

It is not known if Hendren was arrested. CNN affiliate, KMOV, indicated that he was wanted.

St. Louis Circuit lawyer Kimberly M Gardner said on Friday that Hendren could be sentenced to three to ten years in prison.

"I will hold accountable to those who violate the Missouri law, regardless of their profession, public status or life situation," she said in a statement. "Today, as long as it is sad for my staff and myself to file these charges, Katlyn and her family deserve justice and justice."

The victim leaves behind a husband

In his first report on the incident, police said the death was accidental and occurred when an anonymous agent "mishandled" a firearm while they were sitting in the living room.

According to a police statement, a call to an "officer in need of badistance" was launched at 12:56 pm. The third officer informed the dispatcher that he was taking Alix to the hospital.

She was pronounced dead soon after her arrival, the statement said.

Gardner said in a statement that the Missouri road patrol was to help her office conduct an "independent investigation into potential criminal behavior" in connection with the shooting.

The statement notes that the investigation of the prosecutor of the court is part of the protocol during a shootings by the police to ensure transparency.

The police force investigation unit of the police department is also investigating.

Alix had two years of experience in the force, said the SLMPD. She leaves behind a husband, her parents and a sister, among other family members.

Aimee Chadwick, Alix's mother, declined to comment on Saturday for CNN, citing the ongoing investigation. But she spoke Friday to CNN's affiliate, KMOV, about her daughter's love for the job.

"Even during her days off, she would visit her friends there because, as I suppose, when you are a police officer, you have close ties with everyone."

Chadwick said his daughter was "one of a kind".

"There will never be any other Katie," Chadwick said. "She was the most generous, kindest person, if you needed her, she would be there."

SLMPD Commissioner John Hayden called Alix "an enthusiastic and energetic young woman with a promising future".

The St. Louis Police Union released a statement acknowledging that the public wanted to "understand what had happened".

"We, too," said the union. "But for now, we wait, we wonder, and we cry."

In addition to Alix, at least 10 other police officers died in 2019, according to the National Memorial Fund for Law Enforcement Officers. Five died from firearms, three from traffic accidents and two from unspecified causes.

By Ralph Ellis, CNN

© Nine Digital Pty Ltd 2019