COLUMBIA, S.C. (AP) — A mother drowned one of her daughters in their South Carolina home and was trying to kill another child when the oldest daughter was awakened by screams and managed to save her sister, a sheriff said.

Jamie Bradley Brun, 37, was charged with murder and attempted murder after the attack early Friday in their home on St. Helena Island, Beaufort County Sheriff P.J. Tanner said at a news conference.

Brun has talked to investigators and Tanner called it a horrific crime but told reporters he wouldn’t say why Brun wanted to kill her children.

“I’m not a mental health expert. It’s not my job to determine if someone has a mental health problem,” the sheriff said.

Brun’s 16-year-old daughter was asleep around 1:30 a.m. Friday when her 8-year-old sister’s screams woke her up. The sheriff wouldn’t detail how the mother was trying to drown her child, but said the girl’s cries were coming from the bathroom.

The teen went into the bathroom and managed to wrest her sister away from her mother and run to a nearby family member’s house to call 911, Tanner said.

“I’m very proud of her. I think she did an unbelievable job. She defended her family when no one else was available,” Tanner said. “Her courage is amazing.”

Deputies arrived eight minutes later and found Brun and her 6-year-old daughter, who was later pronounced dead at the hospital after CPR by officers and paramedics could not revive her, investigators said.

Brun tried to grab a deputy’s gun as she was arrested and officers shocked her with a Taser to take her into custody, Tanner said.

Brun was being held without bond. Jail and court records didn’t indicate if she had a lawyer.

The 8-year-old did not appear to be seriously injured, and the sheriff said she and her teen sister are being cared for by other family members.

Brun had no arrest record in South Carolina and authorities had been called to the house only once about two years ago after a school employee was concerned about the mental health of one of the children, Tanner said.

“There is a lot more information that we have and a lot more details that we know but at this point we can’t share,” Tanner said.