Celeste Rivas ' death investigation is "complicated" and one of the key undetermined factors is establishing cause of death ... this according to the LAPD Captain leading the investigation.

Captain Scot Williams tells TMZ ... they are working with the L.A. County Medical Examiner to establish Celeste's cause of death. This is critical because up to now, it hasn't been clearly established whether she died from foul play or other causes. In other words, without the M.E.'s findings, it can't be squarely called a homicide, drug overdose, or anything else.

The Captain also tells us ... investigators continue to process the Tesla for evidence -- and that includes possible digital and video evidence. We asked if Tesla cameras captured the person or persons who put Celeste's body in the truc

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