Whitney Houston Cause of Death -- Autopsy Complete, Results Pending Toxicology

The autopsy of Whitney Houston is complete, TMZ has learned, but the results will be deferred pending a toxicology report.

According to our sources, a cause of death cannot be determined until the toxicology results ... which typically takes 4-6 weeks.

A coroner's spokesman says the results are on a security hold, which just means nothing is being released at this time.

As TMZ first reported ... although the cause of death is unknown, it is increasingly looking like Whitney may have fallen asleep or lapsed into unconsciousness in the bathtub and drowned.

We're told Whitney had been drinking the night before and authorities believe a combination of Xanax and alcohol could have fatally sedated her -- although our sources say it is simply too early to draw any firm conclusions.

A rep for the Beverly Hills PD just announced during a news conference -- the preliminary investigation is nearly complete ... pending final results from the L.A. Coroner's Office.