Sacramento police arrested a 29-year-old mother after an investigation found her baby likely died from burns suffered in a microwave oven.
Ka Yang was being held without bail in Sacramento County Jail on suspicion of murder and assault resulting in the death of a child.
She was arrested yesterday, three months after her six-week-old daughter, Mirabelle Thao-Lo, was found dead in the family home on March 17.
Police described the child as suffering "extensive thermal injuries".
Officer Laura Peck said the arrest took so long because investigators had to pinpoint what they believe is the cause of death by looking for other cases involving similar injuries.
They found three in the US, all after children were burned in a microwave: in Dayton, Ohio; Galveston, Texas; and New Kent County, Virginia.
"This is rare. The injuries were obviously very unique," Officer Peck said. "There have been only three other documented cases in the entire country where the injuries were consistent with this case... Those children were also burned and placed in the microwave."
She said police did not know the motive.