Robin Williams’ autopsy found no alcohol or illegal drugs in his body when he killed himself at his Northern California home in August.

The autopsy results released by the Marin County sheriff’s office found that the actor had taken prescription medications but in “therapeutic concentrations”.

The coroner ruled Williams’ death a suicide that resulted from asphyxia.

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Sheriff’s officials have said Williams was found in the bedroom of his home in Tiburon on the morning of August 11.

His death had been preliminarily ruled a suicide.

Sheriff’s Deputy Stewart Cowan said Friday that his office was not releasing further information from the report.

Williams’ wife, Susan Schneider, has said the actor and comedian was struggling with depression, anxiety and a recent Parkinson’s diagnosis when he was found dead by his personal assistant at his Marin County home.

Williams had publicly acknowledged periodic struggles with substance abuse.

He had entered a substance abuse rehabilitation program shortly before his death.

The results of Williams’ autopsy, including toxicology tests, were originally slated to be released on September 20.

Marin County officials later announced a November 3 release date, but the report was further delayed. Toxicology reports routinely take up to six weeks to complete.

* Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14 or Suicide Call Back Service 1300 659 467.