Lisa Marie Presley's cause of death won't be known for some time -- because health officials are waiting on one key thing that'll hopefully explain what took her life.
Sources with direct knowledge tell us Presley's autopsy was done Saturday, but we're told the L.A. County Coroner's Office doesn't have an official COD yet -- because toxicology tests are being done, and the results aren't in yet. No word on when those might come back.
In other words ... the underlying cause of what sparked her medical emergency remains a mystery. The toxicology will expose what may have been in her system at the time of death.
TMZ broke the story ... Lisa Marie went into full cardiac arrest Thursday morning at home after complaining of severe stomach pain only hours before. Family sources tell us Lisa Marie's ex, Danny Keough, performed CPR until paramedics arrived.
First responders were able to get a pulse after administering at least one dose of epinephrine ... but she died at the hospital surrounded by her family. We're told she was pronounced brain dead at one point while on life support, spurring her family to sign a "DNR" ... which eventually led to her having a second cardiac arrest and eventually passing.
Of course, Lisa Marie and Priscilla Presley were both in attendance at The Golden Globes Tuesday where Austin Butler took home the award for Actor in a Drama Motion Picture for his role in "Elvis."
Butler praised both Lisa Marie and Priscilla during his acceptance speech, which nearly brought both women to tears.
Lisa Marie was only 54.
RIP