Jail records show Aaron Dean, 34, was out of custody after posting a $200,000 bond just two days after the incident. Dean also resigned from the force but would have been fired, authorities said.
Police bodycam video showed Dean approaching the door of the home where Atatiana Jefferson, 28, was caring for her 8-year-old nephew early Saturday. He then walked around the side of the house, pushed through a gate into the fenced-off backyard and fired through the glass a split-second after shouting at Jefferson to show her hands.
Dean was not heard identifying himself as police on the video, and Interim Police Chief Ed Kraus said there was no sign Dean or the other officer who responded even knocked on the front door.
"Nobody looked at this video and said that there's any doubt that this officer acted inappropriately," Kraus said.
Sgt. Chris Daniels read a statement after Dean's arrest in which he pledged that the department's major case and internal affairs units were working "around the clock" for justice in the case.
"To the citizens and residents of our city: We feel and understand your anger and disappointment and we stand by you as we work together to make Fort Worth a better place for all of us," Daniels said.
Earlier that day, Jefferson's family had demanded that Dean, a member of the force for 1½ years, be fired and arrested.
"Why this man is not in handcuffs is a source of continued agitation for this family and for this community," family attorney Lee Merritt said, hours before Dean was booked into jail.
Following Dean's arrest, Merritt said the family "needs to see this through to a vigorous prosecution and appropriate sentencing" and added that that "the city of Fort Worth has much work to do to reform a brutal culture of policing."
Police went to Jefferson's home about 2:25 a.m. after a neighbor called a non-emergency line to report a door ajar. In a statement over the weekend, the department said officers saw someone near a window inside the home and that one of them drew his gun and fired after "perceiving a threat."
The video showed Dean shouting, "Put your hands up! Show me your hands!" and immediately firing.
Jefferson was staying up late, playing video games with her nephew, when she was killed, according to the family's attorney.
As for what, exactly, led Dean to open fire, the police chief said: "I cannot make sense of why she had to lose her life." The chief said Dean resigned without talking to internal affairs investigators.
The video included images of a gun inside a bedroom. Kraus said he did not know whether Jefferson was holding the weapon. But he said the mere fact she had a gun shouldn't be considered unusual in Texas.
"We're homeowners in Texas," the police chief said. "Most of us, if we thought we had somebody outside our house that shouldn't be and we had access to a firearm, we would be acting very similarly to how she was acting." Kraus said that, in hindsight, releasing the images of the weapon was "a bad thing to do."
Mayor Betsy Price called the gun "irrelevant."
"Atatiana was in her own home, caring for her 8-year-old nephew. She was a victim," Price said.
Texas has had a "castle doctrine" law on the books since 2007 that gives people a stronger legal defense to use deadly force in their homes. The law was backed at the time by the National Rifle Association and is similar to "stand your ground" measures across the U.S. that say a person has no duty to retreat from an intruder.
Jefferson was a 2014 graduate of Xavier University in New Orleans and earned a bachelor's degree in biology. She was working in pharmaceutical equipment sales and was considering going to medical school, according to the family's lawyer.