Tragic actress Lisa Robin Kelly's passing remains a mystery after her autopsy failed to uncover a cause of death.

The That '70s Show star passed away aged 43 on Wednesday (14Aug13) at a rehabilitation facility in California, just two days after she admitted herself to the clinic in a bid to tackle her long-running alcoholism.

It was believed the star suffered a fatal cardiac arrest, but a spokesperson for the Los Angeles County Coroner's Office reveals a cause of death is still unknown after an autopsy was completed on Saturday (17Aug13).

The coroner is now awaiting toxicology results, which could take several weeks.