Whitney Houston was killed by a combination of Valium, Xanax, and alcohol, an insider tells Radar. All three were found in her system, but the coroner is still waiting for toxicology results before the "major contributing factor of her demise" can be determined, the source says, adding that drowning is almost certainly not to blame: "Her heart stopped beating because her respiratory system was suppressed because of the anti-anxiety meds, and combined with the booze it probably happened very quickly before she was found partially submerged in the bathtub." (In other Whitney news, sources say Bobby Brown is shopping a tell-all about their life together.)