Investigators are focusing on at least five doctors who had prescribed drugs to Michael Jackson, in a bid to unravel the circumstances surrounding the death of the pop star, a Los Angeles newspaper reported on Saturday.

The Los Angeles County coroner's office and the Los Angeles Police Department are being aided in their probe by the Drug Enforcement Administration, the Los Angeles Times said, quoting law enforcement sources.

The California attorney general's office has also offered assistance, the paper said, adding that the agencies concerned have expertise in investigating doctors suspected of improperly prescribing drugs.

But the paper did not disclose any details about the doctors being investigated.

Authorities removed drugs and other medical evidence from the mansion where Jackson was stricken and are trying to determine whether the medications were properly prescribed and whether they played any role in his death, the paper said.

Investigators listed "numerous bottles" of the powerful sedative Diprivan at Jackson's home as one of the most significant clues so far, said the paper.

Some of the bottles were full and others were empty, and none had prescription labels, the paper said, adding that investigators are trying to determine how Jackson got the drugs.

The names on some of the prescriptions were Jackson's pseudonyms, according to the paper.

Diprivan is an extremely potent drug that is supposed to be dispensed by a person trained to administer anesthesia, such as an anesthesiologist or a certified registered nurse anesthetist. The medicine is typically used in hospitals. Experts expressed alarm that it would be used at a private home.

"It's a very dangerous drug if self-administered or administered by someone not trained in airway management and cardiac life support," Ethan Bryson, an assistant professor of anesthesiology at Mt. Sinai School of Medicine in New York, said in remarks published by the paper. "You need to have someone who knows what they are doing when they administer it."

Diprivan surfaced in the Jackson case last week when a nurse, Cherilyn Lee, said the singer complained to her earlier this year about having insomnia and asked her to get Diprivan for him, according to the paper.

Lee said she told Jackson, "this medication is not safe." But the nurse said she never saw him take the drug.

J. Randy Taraborrelli, a Jackson biographer, said the singer was in pain in recent years and had trouble sleeping. "This was a person who would have paid anything -- done anything -- to get a decent night's sleep," he was quoted as saying. "I'm not sure Michael Jackson got a decent night's sleep without medicinal aid."

It remains unknown whether prescription drugs played any role in Jackson's death. The coroner's office is awaiting the results of toxicology testing before determining the cause of Jackson's death.

Source: Xinhua