The officer was served a written administrative complaint Sunday, placed on detached duty and stripped of his badge and firearm, Kraus said.
"My intent was to meet with him today to terminate his employment with the Fort Worth Police Department, however, the officer tendered his resignation this morning before we met," Kraus said.
Kraus said the officer's name is Aaron Dean. If Dean had not resigned, he would have been fired for several policy violations, including the department's use of force and de-escalation policies, and unprofessional conduct, Kraus said. The officer, who was dishonorably discharged from the force, may face criminal charges, he said.
The department has presented a preliminary case to the FBI to review the officer's actions for possible civil rights violations, Kraus said.
"None of this information can ease the pain of Atatiana's family but I hope it shows the community that we take these incidents seriously," he said.
Dean was hired in August 2017 and commissioned as a licensed officer in April 2018, Kraus said.
'There is nothing that can justify what happened,' mayor says
Fort Worth Mayor Betsy Price said Monday the killing of Jefferson was unjustified.
"I'm so sorry. On behalf of the entire city of Fort Worth, I'm sorry," Price told reporters. "To Atatiana's family, it's unacceptable. There is nothing that can justify what happened on Saturday morning. Nothing."
Two seconds after he shouted commands outside a house, a Fort Worth police officer opened fire into what looks like a dark room.
Moments later, Atatiana Koquice Jefferson died in the bedroom of her own home with her 8-year-old nephew nearby.
Now Texans are outraged over the death of another black person killed at home by a white police officer.