Carrie Fisher died from sleep apnea and a combination of other factors, but investigators have been unable to pinpoint an exact cause, say coroner's officials.

Among the factors that contributed to Fisher's death was the build-up of fatty tissue in the walls of her arteries, the Los Angeles county coroner's office said on Friday.

The Star Wars actress showed signs of having taken multiple drugs, but investigators could not determine whether they contributed to her death in December.

The manner of her death would be listed as undetermined, officials said.

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They have not responded to a request for additional details about whether a full autopsy report and toxicology results are available.

Sleep apnea is a condition where a person's breathing pauses during sleep. The pauses may be brief or last several minutes, according to information from the National Institutes of Health.

Fisher, 60, suffered a medical emergency on an international flight on Dec 23. Her mother, veteran movie star Debbie Reynolds, died the following day.

They were laid to rest together at Forest Lawn-Hollywood Hills, a cemetery where numerous celebrities are buried.

Fisher's brother, Todd, said he was not surprised by the results.

He added that his family did not want a coroner's investigation into his sister's death.

"We're not enlightened. There's nothing about this that is enlightening," he said.

"I would tell you, from my perspective that there's certainly no news that Carrie did drugs," Mr Fisher said.

He noted that his sister wrote extensively about her drug use and that many of the drugs she took were prescribed by doctors to try to treat her mental health conditions.

Fisher long battled drug addiction and mental illness. She said she smoked pot at the age of 13, had taken LSD by 21 and was diagnosed as bipolar at 24. She was treated with electroshock therapy and medication.

"I am not shocked that part of her health was affected by drugs," Mr Fisher said.

He said his sister's heart condition was probably worsened by her smoking habit, as well as the medications she took.

"If you want to know what killed her, it's all of it," he said.

Mr Fisher said it was difficult to blame doctors who treated his sister because they were trying to help her.

"They were doing their best to cure a mental disorder. Can you really blame them?" he said. "Without her drugs, maybe she would have left long ago."

Fisher made her feature film debut opposite Warren Beatty in the 1975 hit "Shampoo."

She also appeared in "Austin Powers," "The Blues Brothers," "Charlie's Angels," "Hannah and Her Sisters," "Scream 3" and "When Harry Met Sally ..."

She will reprise her role as Leia Organa in the eighth installment of the core "Star Wars" franchise, "The Last Jedi," which will be released in December.