The St. Louis prosecutor is raising concerns about whether police tried to block drug and alcohol testing of two on-duty officers following the fatal shooting of Officer Katlyn Alix.

Charging documents say Alix and Officer Nathaniel Hendren were taking turns pulling the trigger of a gun loaded with one bullet while pointing it at each other when Alix was shot in the chest. Hendren has been charged with involuntary manslaughter.

The incident took place Thursday at an apartment where Alix was off-duty and with two on-duty male officers.

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In a letter to Police Chief John Hayden and Public Safety Director Jimmie Edwards that was obtained by CBS affiliate KMOV-TV, Circuit Attorney Kim Gardner said urine and breath tests were performed on the male officers, but not a blood test. She wrote that the way the testing was performed "appears to be an obstructionist tactic."

Gardner's letter said there was "probable cause at the scene that drugs or alcohol may be a contributing factor in a potential crime."

This undated photo released by the St. Louis Police Department shows officer Katlyn Alix. St. Louis Police Department

At a press conference Tuesday, Edwards defended the St. Louis Police Department and the chief's actions.

"Our officers are not obstructionist," he said, KMOV-TV reported.

"We have to be careful and we have to be fair and we should not vilify the entire department based on what has transpired here," he said.

He said it's the role of the police department, not the prosecutors, to process a crime scene.