SAN DIEGO (CNS) - A woman accused of fatally shooting her 74-year-old great-aunt after being let in to the victim's Tierrasanta home pleaded not guilty Wednesday to murder charges.

Tiffany Nicole Burney -- held on $2 million bail -- faces 50 years to life in prison if convicted in the death of Daisy Mae Hayes.

Deputy District Attorney Roy Lai said Burney shot her great-aunt four times in the face from close range in the early morning hours of Dec. 14.

Lai said Hayes was at a hospital the previous day taking care of her husband of 55 years, who recently had back surgery.

Burney, 23, was apparently let into the victim's home early the next morning because there were no signs of a struggle, the prosecutor said.

Investigators are still trying to figure out exactly what happened inside the residence, but they do know Burney fired five shots at her great-aunt, with four hitting the victim in the face and head, Lai told reporters outside court.

Hayes' body was found the night of Dec. 14 in the living room of her Gabacho Drive home after relatives were unable to reach her and went to the residence to check on her welfare.

Burney, the granddaughter of the victim's sister, was arrested Saturday in La Mesa.

The prosecutor said the defendant lived a transient lifestyle and police had trouble locating her for three days after the murder.

He said Burney was an "angry, frustrated and unstable" individual who had access to an unregistered revolver and used it to kill her great-aunt. Lai said Burney made statements to police following the murder, but those statements weren't revealed.

"Whenever we're dealing with someone who's willing to kill another individual, whether it be a family member, it's safe to say that of course there is frustration, there must be some sort of rage, some sort of instability," Lai told reporters. "But other than that, it does take an unstable individual to kill another individual."

According to court records, Burney was the subject of two mental health cases filed last year and also filed a lawsuit against the county mental health agency in August 2010.

Lai was asked about the possibility of a mental health defense.

"We'll deal with that when we cross that bridge," Lai said.

He said the focus should be on the victim and her family.

"There is a 74-year-old matriarch of a very large family. She had four children ... three daughters," the prosecutor said. "This is an individual who had been married to her husband, was taking care of her husband at a hospital and had been married to him for about 55 years. They had lived in that same house for over 30-plus years.

"And here we have, by all accounts a loving individual, a church-going victim, and Daisy Mae Hayes was killed so close to Christmas. And our thoughts and prayers should go out to the family, because at this point their holidays are ruined."

Burney will be back in court Jan. 11 for a status conference.