A Sacramento baby who was allegedly killed by her mother in a microwave oven died of "extensive second-, third- and fourth-degree thermal injuries," coroner's officials confirmed Wednesday.

The Sacramento County coroner's office also officially listed the death of 6-week-old Mirabelle Thao-Lo as a homicide.

Mirabelle's mother, 29-year-old Ka Yang, was arrested Tuesday and charged with homicide. Investigators later said they believed Yang had burned Mirabelle in the microwave oven.

Mirabelle was found dead at her home on March 17. Firefighters were initially told an adult holding the baby had suffered a seizure and dropped her. Because of the severe burns the infant had on her body, however, firefighters called police.

Investigators spent the next three months investigating the cause of the unusual burns, and determined they were similar to documented cases in which children had been burned by microwaves.

"Based on that research and the autopsy results, detectives believe the injuries occurred as a result of a child being burned in a microwave," the Sacramento Police Department said in a statement.

Yang has three other children who neighbors said were taken into protective custody.

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Allegations that mother killed baby in microwave oven horrify neighbors

-- Kate Mather

Photo: Ka Yang. Credit: Sacramento Police Department