32-year-old Lorna Valle is charged with two counts of murder, with the special circumstance allegations of multiple murders and assault on a child causing death.
Valle was originally charged for the drowning death of her one-year-old daughter, Lindsay, and the attempted murder of her 5-year-old daughter, Marian.
However, that charge was later changed to murder when Marian had to be taken off life support a few weeks ago.
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Valle is being held without bail. Prosecutors have not decided whether to seek the death penalty.
Police were called to the family's home in the 900 block of W. 50th Street on the morning of Feb. 15, after the children's father, Jose Lopez, discovered the girls.
Lopez came home from the grocery store to find his wife trying to drown the girls, police said.
A neighbor told KTLA that when Lopez returned, he heard shouts of "My babies! My babies!"
Dina Ceballos, who lived across the street, told the Los Angeles Times that she heard the father say, "If I didn't get out, my girls would still be alive."
"He was screaming and saying, 'Why didn't she kill herself instead of trying to kill my girls?'" Ceballos said.
The girls were found in the bathroom of a small converted garage where the family lived, LAPD Cmdr. Andy Smith said.
He said the children were "not conscious and not breathing when the fire department arrived."
Neighbors who witnessed the aftermath told KTLA the toddler was purple and "in really bad shape."
They also said they saw Valle with a look they'll never forget: "Blank face," one woman said. "She wasn't saying nothing. No expression, no crying, no nothing."
Both girls were rushed to California Hospital, where one-year-old Lindsay was pronounced dead.
Lopez later made the decision to remove Marian from life-support and donate her organs.
Valle reportedly has a history of mental illness. She was hospitalized for psychiatric evaluation at Harbor-UCLA Medical Center.
"We believe she is responsible for what happened to these two children," Cmdr. Smith said.
A neighbor said the family is from Guatemala, and moved to South L.A. about ten years ago.
She said the family was nice and hard-working, and the girls often played in the yard.
But, she added, the mother appeared to be depressed and possibly suicidal recently.
"A neighbor said she told her that if anything happens to her, she would leave the girls to the father or to them," she said.
"She was asking for psychological help and she didn't receive it. It's a sad story. It's a sad ending. She needs help," the neighbor said.
A family cousin, Luis Padillas, told KTLA that Valle had sought professional help, but was turned away.
He said she was an illegal alien and wanted to go back home. Valle was very depressed, according to Padillas.
He said they took her to the hospital, but she was not treated because she had no papers.
"We need a lawyer that can represent us because of the negligence of the hospital," he said in Spanish.
Padillas did not specify which hospital or the date Valle went seeking treatment.