Prosecutors say Hendren and his partner, who were on duty, were hanging out at Hendren's house after midnight last Thursday with Alix, who was off duty.
According to charging documents, Hendren and Alix reportedly took turns holding a revolver loaded with a single bullet.
The documents said at some point "the defendant took the gun back and pointed it at the victim, pulled the trigger causing the gun to discharge. The victim was struck in the chest."
Police initially labeled the shooting accidental. The circuit attorney Kimberly Gardner has criticized the St. Louis Police Department with obstructing the investigation.
"The accusation lodged by the circuit attorney, to say the least, was both offensive and assaulting to the force investigative unit and myself," said St. Louis Police Commissioner John Hayden.
Alix was buried Wednesday. Her mother, Aimee, described her love of service
"She always told me, 'Mom, if I died, I did doing something I loved to do.' But this does not make me feel better," she said.
Hendren was ordered to surrender any firearms in his possession and his partner has been placed on administrative leave. The St. Louis police department said effective immediately, supervisors will have to verify the location of their officers every hour using GPS.