Pedestrians walk past an image of singer Whitney Houston displayed at the… (Mel Evans, Associated Press )
The investigation into the death of Whitney Houston is shifting to a new phase, with officials focusing on the prescription drugs found in her hotel room and who prescribed them to her.
Investigators are expected in the next few days to serve subpoenas on the doctors, as well as the pharmacies where Houston obtained the prescriptions, as they try to determine her cause of death, according to a source with knowledge of the case.
Authorities collected several bottles of drugs from Houston's suite at the Beverly Hilton Hotel, where she was found dead Saturday. But officials have said the amounts of drugs did not seem unusually large, leaving it unclear whether the medications had anything to do with the singer's death. Officials are waiting for the results of toxicology tests on Houston's body.
Photos: Whitney Houston, 1963-2012
The source would not discuss specifics of the case but said it was standard practice to examine whether the drugs were dispensed properly and if there was any indication that she was receiving too many prescriptions. The source spoke on the condition of anonymity because the case is ongoing.
Dave Campbell, a retired captain from the coroner's office, said investigators would count the tablets in each container and compare them against the date of the prescription to see if the person was taking the correct dosage.
"Sometimes you find other medications inside" the bottles, he said.
Defense attorney Ellyn Garofalo, who won acquittal for a physician charged with over-prescribing drugs to Anna Nicole Smith, said investigators were probably going to be looking at several specific areas.
They will compare the amounts of prescription medications gathered from Houston's room with the amounts of medication that were dispensed. They will look at which pharmacies dispensed the drugs and which doctor or doctors prescribed them. That information could be compared against the prescribing history of one or more doctors who treated Houston.
A red flag would be a single doctor prescribing enormous amounts of medication, Garofalo said.
After Michael Jackson died in 2009, authorities spent months looking at bags full of prescription drugs found at his home. Prosecutors charged his doctor, Conrad Murray, in connection with the star's death.
Investigators will probably also use a state-created database with more than 100 million entries for controlled substances prescribed in California. The database has been used in past cases to determine the amount of drugs patients were receiving and how much doctors were prescribing.
Houston's death is being investigated by the L.A. County coroner's office and the Beverly Hills Police Department. Police said they have no plans to launch a criminal investigation, and the coroner's office said it won't have a final cause of death until toxicology results come back.
Tracking Houston's medications could take time. L.A. County sheriff's Sgt. Steve Opferman, who oversees a prescription drug task force but is not involved in the Houston case, said "celebrities often get their prescription drugs from doctors who are more than willing to give them what they want and sometimes using members of their entourage." Celebrities say they do this to protect their privacy.
Garofalo said building a case against a doctor or pharmacist is difficult.
"It's a very high bar to prosecute a doctor who has wide latitude to prescribe medication and deniability because there are clear directions about how prescription medications should be used," she said.
Experts also stressed that it's difficult to draw conclusions until the tests are done.
Full coverage: Whitney Houston, 1963-2012
In 2010, actor Corey Haim died unexpectedly. The attorney general's office found that in the months before his death, Haim, 38, got 553 painkiller pills. But the coroner's office determined that Haim died of respiratory distress related to pneumonia with the presence of an enlarged heart and narrowing of blood vessels. The low level of eight drugs, some obtained by prescription, did not contribute to his death.