Carrie Fisher Cause of Death 'Not Obvious'

Carrie Fisher's death certificate has been issued, TMZ has learned, but it does not list a cause of death because it's not clear what killed her.

The L.A. County Coroner's Office performed an autopsy Friday it has "deferred" reaching a conclusion on the cause of death. Coroners officials tell us they need to do more tests to determine what ended the 60-year-old actress's life. An official adds, the cause of death is "not obvious."

The reason a death certificate was issued before the cause of death could be determined ... the family can't bury the body without a death certificate, so it's routine for the Coroner to issue death certificates before cause of death can be determined.

TMZ broke the story, Fisher's heart stopped one week ago on a United flight that was 15 mins away from landing in Los Angeles. The situation was so desperate she did not have a pulse for several minutes before a passenger was able to revive her with CPR.

She was taken to The UCLA Medical Center but did not get better ... she died Tuesday after lingering on life support.