DENVER (KDVR) – A Denver Police officer whose stolen semi-automatic rifle was used in the murder of 21-year-old Isabella Thallas has resigned, FOX31 has learned.

Sgt. Dan Politica submitted his resignation Feb. 13 and worked his last day with the department a month later, according to a resignation letter obtained by FOX31.

Isabella’s father, Joshua Thallas, said the resignation raises even more questions about what happened that day.

“If there was no wrong-doing, why should he leave his job?” he said. “Not a job: why should he leave his career? And hire a private counsel? Those draw up huge red flags of what the heck has gone on.”

As she walked her dog in the Ballpark neighborhood, Isabella was killed on June 10 with a stolen weapon that was never reported missing.

Denver Police later said the gun belonged to a Denver police sergeant who was friends with her killer. They said it was not a police-issued weapon.

Thallas said the city did not inform the family of Politica’s resignation.

“We’re just looking for an answer. It’s been almost a year, we’ve played by the rules, we’ve stayed quiet when asked to be quiet by the DA, and attorneys, and it’s time to have a voice. It’s time to know why. Not just us as a family and friends. Everybody wants to know what the heck has gone on,” Thallas said.

Michael Close, the man accused of killing Isabella, has pleaded not guilty by reason of insanity.

Close is accused of firing at least 24 shots from the high-powered rifle, killing Isabella and injuring her 27-year old boyfriend, Darian Simon, who was shot twice in his legs but survived.

The circumstances surrounding the homicide led a new law named named after Isabella. It requires gun owners to report to law enforcement within five days of realizing a firearm is lost or stolen.