Two bodyguards, a hairdresser and stylist were believed to have been in the singer's room at the time and discovered her body, with her head under the water, after becoming concerned that she had been in the bathroom for more than an hour.
Prescription drugs, but no illegal drugs, were found in the suite. Houston was known to take Xanax, a drug to ease anxiety which can cause drowsiness, before performances. Anti-anxiety drugs Valium and lorazepam were also believed to have been found.
An autopsy will establish whether the singer fell asleep and drowned, died from an overdose, or from some other cause. Paramedics tried to resuscitate her for 20 minutes but she was pronounced dead at 3.55pm (11.55pm GMT) on Saturday.
On Thursday, two days before her death, witnesses said Houston had displayed erratic behaviour, including doing handstands by the Beverly Hilton Hotel pool, and wandering aimlessly around the lobby.
Later that night she was seen looking dishevelled at a Hollywood nightclub, drinking tequila, and video showed her performing there briefly in a faltering voice. On Friday night she was seen "partying hard" in the hotel bar with friends.