When questioned by police Monday, Brennan admitted she had smothered her mother with a pillow, according to KNXV.
"Responding officers then went inside the apartment and located an elderly female deceased on the bed," Hoster told the television station.
Mike Bennett, who lives in the apartment complex, told KNXV that Simmons was "a sweet lady," but Brennan was "a little sketchy."
Bennett said he was surprised to hear Brennan’s confession.
"They handcuffed her and sat her down on the ground, and then they wouldn't let me out that door because they taped it all off," Bennett told KTVT. "She said, 'I did it, I smothered her with a pillow,' and I'm like, 'Am I hearing this?'"