Investigators concluded that 32-year-old Rebecca Zahau bound her own hands and feet and hung herself naked off a second-floor balcony on July 13.
Her family says they don't believe she killed herself.
Authorities said Zahau left a message in black paint on a door but didn't reveal the message's content. They stopped short of calling it a suicide note. DNA from the rope around her neck that was tied to a bed footboard, and foot impressions on balcony dust were also tied to Zahau, investigators said.
Dr. Jonathan Lucas of the San Diego County Medical Examiner's Office, who conducted the autopsy, said he was confident in the findings.
Zahau's death came two days after her boyfriend's 6-year-old son Max Shacknai fell down some stairs while under her care. The boy succumbed to his injuries days later.
Max's father, Jonah Shacknai, is an Arizona pharmaceuticals magnate who owns the 27-room waterfront mansion. Zahau was his girlfriend of two years, and Max was his son from a marriage that ended in divorce in 2008.
Max's death has been ruled as an accident.
The Associated Press contributed to this story.