The mother of a "pretty little girl" who police say was tortured and murdered by her father and stepmother still doesn't know her daughter is dead, officials said.

Four-year-old Summer Lytle apparently drowned in a bathtub Saturday night, where her spiteful stepmother left her sitting alone in urine-stained clothing while she went into the kitchen to cook the girl's father an anniversary dinner, court documents said.

Investigators said the girl's biological mother lives somewhere in rural Western Washington, and authorities were hoping they could locate her Tuesday and bring her back to Spokane to help in the investigation.

According to the documents, the child also had injuries from beatings with belts and spoons and had human bite marks on her body, and her death ended a pattern of abuse that included a dog's shock collar and burns to her face, the court documents said.

"This is the worst case I've ever seen and my detectives feel the same," said Sgt. Joe Peterson of the Spokane Police Department's major crimes unit. "It's probably the worst case of abuse that any of us in hundreds of combined years in this unit have ever seen."

Summer's father, 28-year-old Jonathan D. Lytle, and her stepmother, 32-year-old Adriana L. Lytle were ordered held in Spokane County Jail on a $500,000 bond each on charges of homicide by abuse.

Jonathan Lytle took Summer to Deaconess Medical Center on Sunday with bruises from head to toe, according to court documents.

Child abuse investigators had opened a case file on the Lytle family in October, but found no cause for action, according to spokeswoman Kathy Spears with Washington Department of Social and Health Services.

"Usually, we close a case when the threat of abuse or neglect is diminished," Spears said.

She said the agency investigated a report that Summer was being neglected in January 2006, but that social workers had received no specific reports of physical abuse involving the preschooler, the Spokesman Review newspaper reported.

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"Nobody called in saying they had seen this child covered with bruises," Spears said, adding: "I'm not going to second-guess what happened with this investigation."

An autopsy began on Monday to determine what killed Summer.

"There are extensive injuries, so the autopsy is going to take a long time," Peterson said.

During police interviews, both Jonathan and Adriana Lytle admitted that they subjected Summer to various punishments, the Spokesman Review reported.

"This included the use of a dog's training shock collar, beatings with leather belts, beatings with spoons and making Summer tear pizza boxes into tiny pieces," according to court records.

Neighbors said they were shocked by the allegations against the couple.

"She was a pretty little girl," one neighbor said. "This was too bad."

A second child, an 8-month-old boy, was taken from the home and placed in foster care.

The Associated Press and Spokesman Review contributed to this report.