The arrest warrant issued for former Fort Worth police officer Aaron Dean says Atatiana Jefferson was holding her pistol when she shot and killed by the officer around 2:30 a.m. Saturday morning. Dean has been charged with murder over the incident.

"She heard noises coming from outside, and she took her handgun from her purse," Jefferson's 8-year-old nephew told police, according to the warrant. The 28-year-old woman had been playing video games with her nephew before the fatal officer-involved shooting, Reuters reports.

The officers did not knock on the front door of the home or announce they were police before Dean fired his weapon, according to Fort Worth Police Chief Ed Kraus..

"It makes sense she would have a gun if she felt threatened or if there was someone in the backyard," Kraus said.

The officers believed they were responding to an "open structure" call, which is handled differently than a call for a welfare check, the Kraus said. 

WFAA TV reports Kraus said the call changed somewhere between the non-emergency call taker and dispatchers and the department was working to learn why.