Circuit Attorney Kim Gardner later sent a letter to the police department, accusing them of using an "obstructionist tactic to prevent us from understanding the state of the officers during the commission of this alleged crime."
She alleged that the police department failed to take blood draws on the involved officers, when "drugs or alcohol may be a contributing factor in a potential crime."
Gardner also said that police were too hasty to call the incident accidental. In the hours after Alix's killing, St. Louis Police Chief John Hayden called it an "accidental discharge of a weapon.”
At the time, Hayden said commanders had been ordered to keep a closer eye on officers in the field. He also angrily responded to Gardner's charges.
"Let it be crystal clear that the accusation launched by the circuit attorney to say the least was both offensive and insulting to the force investigative unit and myself," Hayden said.
Alix is survived by her husband, mother, father and sister. The family in October filed a wrongful death lawsuit against Hendren, the officer he was with, the police sergeant on duty, and the City of St. Louis, according to KSDK.
"This world lost a lot," her father, Ron Alix, said last year. "When we lost Katie, we lost a lot."
The lawsuit alleged that Hendren had suffered from anxiety, depression, PTSD, and suicidal ideation and had "a history of acting recklessly with firearms, forcing previous girlfriends to play 'Russian roulette' and engage in other sexual activity that involved firearms."
The suit said the city was negligent when it hired Hendren about a year before the shooting because it should have been aware of his past.
Before pleading guilty on Friday, Hendren was under house arrest on a $100,000 bond. He was placed on administrative suspension, which is always unpaid, after the shooting. Later, his peace officer license was later suspended, meaning he cannot work as a police officer.
Elisha Fieldstadt is a breaking news reporter for NBC News.