Katlyn Alix's colleague has been charged with involuntary manslaughter and armed criminal action.

The cry rang out into the empty, bitterly cold streets of St. Louis just before 1 a.m. on Thursday. "Oh, my God!" a neighbor told the St. Louis Post-Dispatch she heard. "Somebody help!"

Neighbors peeked out from their two-story red-brick homes a few blocks from the banks of the Mississippi River and saw a sight too familiar in a city rattled with gun violence: Two men dragging a bleeding woman into a vehicle and then speeding away.

But then they noticed something odd. The vehicle was marked with the St. Louis Police Department logo. And the two men were officers.

In fact, the grievously wounded woman, who soon died at a hospital was a police officer, too: Katlyn Alix, a 24-year-old Army veteran with two years on the force. And St. Louis authorities soon said she was killed by a colleague.

At first, police called it an "accidental" killing after an officer "mishandled" a gun. But St. Louis prosecutors on Friday evening came to a different conclusion. The officers, they say, were playing a deadly game - pointing a gun with one round in the chamber at each other and then pulling the trigger.

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.

"This is a devastating incident for Katlyn Alix's husband, family, parents and for our entire community," Gardner said in a statement. "I will hold people accountable regardless of their profession, public status or station in life."

The charges bring a shocking end to a day of puzzling questions after Alix's death. Before Gardner announced charges, reporters had tried to sort through a series of baffling queries about the case: Why were the two male officers, who were supposed to be patrolling another part of town, inside a home with Alix, who was off-duty? Why was one of those officers handling a revolver that reportedly wasn't a service weapon? And most important, how could a highly trained officer accidentally fire the gun straight into Alix's chest?

"She always told me, 'Mom, if I die, it's doing something I love to do,' " Aimee Chadwick, her mother, told KMOV. "So, it made me feel a little better. But this does not make me feel better."

The bizarre shooting comes at a difficult time for the department, which is already under national scrutiny after four St. Louis officers were indicted by a federal grand jury in November in the brutal beating of a 22-year police veteran who had been posing as a protester during 2017 demonstrations over a fatal police shooting. In text messages later obtained by investigators, some of the officers involved had joked about pummeling protesters.

In Alix's case, the first radio call for help came into police at 12:56 a.m. on Thursday, Police Chief John Hayden later told reporters gathered outside Saint Louis University Hospital. One of the two male officers involved in the shooting issued an "officer in need of aid" plea as they raced to the hospital with Alix in the back of their vehicle.

Hendren, who had about a year on the force, has been identified as one of those officers; his partner, who has not yet been named, had about two years of service in the department, the Post-Dispatch reported.

The two officers, who were supposed to be on duty in a district a few miles away, went to a home in the city's Carondelet neighborhood where one of the two officers lives. Alix, who regularly worked with the two men and was "close" with them, the Post-Dispatch reported, soon joined the officers in the living room.

In the charging documents, prosecutors now say that Hendren and Alix began playing with a revolver. First, prosecutors say, Hendren emptied the chamber, put one bullet back in and spun the cylinder. He pointed the gun away and pulled the trigger. Nothing happened. Alix then took the gun, pointed it at Hendren and pulled the trigger. Again, nothing happened.

Then, prosecutors say, Hendren took back then gun, aimed at Alix's chest and fired. This time, the chambered round found its mark.

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.

The two men then rushed Alix to the hospital, where she was pronounced dead. According to the Post-Dispatch, at the hospital Hendren head-butted the window of a police SUV hard enough to shatter the glass and had to be hospitalized for his injuries.

In a statement sent to reporters later that morning, St. Louis police called the shooting an accident, but two investigations quickly launched into the case: one by the St. Louis Police Department and another by the St. Louis Circuit Attorney's Office with the Missouri State Highway Patrol.

The probe came to a swift end with charges against Hendren announced at 7 p.m. on Friday.

"As much as it saddens my staff and me to file these charges, Katlyn and her family deserve justice," Gardner said in a statement.

An attorney representing Hendren in the internal investigation at the department has not commented to local media on the charges, the Post-Dispatch reports.

Alix's friends described her to local media as a nails-tough fitness buff who excelled as a police officer.

"She could lift ungodly amounts of weight," Taylor Rumpsa, a police dispatcher and friend, told the Post-Dispatch. But Rumpsa described a softer side as well and said Alix loved to interact with children she met on the job. "She loved kids, and she'd try to shoot hoops with them, but she wasn't very good at it," she said.

Alix graduated from high school in nearby Wentzville, Missouri, and signed up for the Army when she was 17, her mother said. "I signed for her, because that's what she wanted to do," Chadwick told KMOV.

Alix served six years in a military police unit, her mother said, before joining the force in St. Louis. In October, she married another St. Louis police officer, who was not present at the home during the shooting, the Post-Dispatch reported.

"Her whole life," her mother told KMOV, "was the police department."