Rebecca Zahau, the girlfriend of San Diego billionaire Jonah Shacknai, didn't kill herself, her family says. They believe she was murdered.

Zahau's body was found nude and hanging from a balcony on July 13 at Shacknai's 27-room home, the historic Spreckels Mansion in Coronado. Shacknai is chairman and chief executive of Medicis Pharmaceuticals. Her death came two days after Shacknai's 6-year-old son, Max, who was under her care was seriously injured in a fall down the stairs. Max later died.

Her death has been ruled a suicide, but her family says she was murdered and they told RadarOnline.com they believe they know who did it.

Becky did not kill herself, someone killed her, Snowem Horwath, Zahau's sister, told RadarOnline.com. Time will reveal who killed Becky. I think I know who killed my sister. We are not stupid; she never had any enemies.

Horwath didn't say who she believes killed her sister, but disputed the evidence the police told her family they used to rule the death as suicide.

They do not have enough evidence to say it is a suicide, it is such a big lie, Horwath told RadarOnline.com. She noted also that the family felt that there were many holes in their theories.

Things they said did not match and they said they had a text message on her phone that showed that she was very unhappy, but they didn't even say if it was from her or from someone else, or if someone could have put it there, Horwath said.

Another sister, Mary Zahau-Loehner, on Thursday said she found San Diego County sheriff's investigators unconvincing when they explained their findings during a visit to her home in St. Joseph, Mo.

It doesn't add up, she told the media. Nothing adds up.

The family wants further investigation into how Zahau, 32, died.

San Diego County Sheriff's Department spokeswoman Jan Caldwell declined to comment, but Sheriff Bill Gore plans a news conference Friday.

Zahau-Loehner said she's not convinced because she spoke with her sister the night before her body was found and there was no indication that she planned to take her own life. Zahau had been seeing Shacknai for two years.

Zahau-Loehner also said investigators told her they found no suicide note. They did however, share text messages on her phone from months earlier about issues between her and Shacknai's children. There have been reports from local media that Zahau may have grown depressed because she was reportedly watching the boy when he fell.