Now, Officer Nathaniel Hendren, 29, has been charged with involuntary manslaughter and armed criminal action, both felony charges, according to documents released late on Friday by Kimberly Gardner, the St. Louis circuit attorney.
Hendren's unnamed partner told prosecutors that he'd told Alix and Hendren that "they shouldn't be playing with guns and that they were police officers", prosecutors say; he says he was leaving the apartment in disgust when the shot rang out.
A probable cause statement from police offered a chilling account of the risky game that led to her death, describing how Alix and Hendren were playing with guns.
"The defendant emptied the cylinder of the revolver and then put one cartridge back into the cylinder", the statement said. Next, per the statement, Alix took her turn, though she reportedly pointed the gun at Hendren.
Hendren then took the gun back from Alix and pulled the trigger, shooting Alix in the chest, police said.
On the third shot, Hendren pulled the trigger while pointing the gun at Alix, and the gun discharged, the statement reads.
The two male officers drove Alix to hopsital, where she died of her injuries.
Hayden, in a statement, says Alix was an "enthusiastic and energetic young woman with a bright future ahead of her".
The shooting happened around 1 a.m. Thursday when two on-duty male officers went to one of their homes during their shift.
At first, police called it an "accidental" killing after an officer "mishandled" a gun.
Flags fly at half-staff in front of the St. Louis Police Officers Association, following the shooting death of a police officer by another officer. Alix died from a gunshot wound to the chest.
A police sergeant investigating the shooting said Hendren's partner claimed he was on the way out when the shot was sacked.
A spokeswoman for St. Louis Circuit Attorney Kim Gardner says the Highway Patrol often assists investigations initiated by the prosecutor's office.
Hendren has been a police officer for about a year.
"I have said this many times before; I will hold people accountable who violate Missouri law regardless of their profession, public status or station in life", the statement continued.
An off-duty USA cop has been pronounced dead after she was shot in the chest during a game of Russian roulette with her colleagues.
Alix, who was a married military veteran, graduated from the St Louis Police Academy in January 2017.
"Today, as much as it saddens my staff and me to file these charges, Katlyn and her family deserve accountability", Gardner said today in a written statement.
Police tape remains at the scene of a shooting in St. Louis on Thursday. "We don't expect this to happen. So do we. But for now, we wait; we wonder; and we weep", he said.