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The homicide ruling was based on forensic tests that found the anesthetic propofol combined with at least two sedatives to kill Jackson, a law enforcement official told The Associated Press, speaking on condition of anonymity because the findings have not been publicly released.
The finding does not necessarily mean a crime was committed. But it does help prosecutors if they decide to file criminal charges against Dr. Conrad Murray, the Las Vegas cardiologist who was caring for Jackson when he died June 25 in a rented Los Angeles mansion.
"It is an easier prosecution when the medical examiner calls it a homicide," said Dr. Michael Baden, the former chief medical examiner in New York City, who is not involved in the Jackson investigation.
Through his lawyer, Murray has said he administered nothing that should have killed Jackson.
Murray told investigators that at the time of Jackson's death, he had been trying to wean the entertainer off propofol. The doctor said he'd been treating Jackson for insomnia for about six weeks with 50 milligrams of the drug every night via an intravenous drip, a search warrant affidavit said.
Murray said he feared Jackson was becoming addicted to the anesthetic, which is supposed to be used only in hospitals and other advanced medical settings.
Murray told police he was present when another Las Vegas anesthesiologist, Dr. David Adams, sedated Jackson with propofol at an unnamed cosmetologist's office "sometime between March and April of this year," according to the affidavit.
It was about the time Jackson requested that Murray become his personal physician for a European tour, the affidavit said.
A woman at Adams' office in Las Vegas referred calls Tuesday to his lawyer, Liborius Agwara, who did not immediately respond to messages seeking comment.
Nevada State Board of Osteopathic Medicine records show Adams has been licensed in the state of Nevada since 2002, and there are no reports of malpractice and no disciplinary issues against him. Ongoing investigations are confidential unless a licensee is named in a formal administrative complaint.
The affidavit was unsealed in Houston, where Los Angeles police took materials from one of Murray's clinics last month as part of their manslaughter investigation. Manslaughter is homicide without malice or premeditation.
The affidavit says Murray told detectives that two days before Jackson's death, he had lowered the propofol dose to 25 milligrams and added the sedatives lorazepam and midazolam, a combination that succeeded in helping the pop star sleep. The next day, Murray said, he cut off the propofol and Jackson fell asleep with just the two sedatives.
But on June 25, Murray said, he tried unsuccessfully to make Jackson sleep with a series of drugs that included a 10-milligram tablet of Valium and repeated injections of two milligrams of lorazepam and two milligrams of midazolam.
When the combination didn't work, he said he gave in to Jackson's "repeated demands/requests" for propofol, which the singer called his "milk," according to the affidavit.
Around 10:40 a.m., Murray administered 25 milligrams of the white-colored liquid - a relatively small dose - and finally Jackson fell asleep.
Murray remained with the sedated Jackson for about 10 minutes then left for the bathroom, the affidavit said. Less than two minutes later, Murray returned and found Jackson had stopped breathing.
Cell phone records show three separate calls from Murray's phone between 11:18 a.m. and 12:05 p.m., the affidavit said. Murray said one call was to Jackson's personal assistant, Michael Amir Williams, saying he needed help to deal with an emergency in Jackson's top-floor bedroom. It's not clear who else received calls. Murray told authorities he was administering CPR during that time.
In a statement posted late Monday on his firm's Web site, Murray's attorney Edward Chernoff questioned the timeline as depicted in the affidavit, calling it "police theory."
"Dr. Murray simply never told investigators that he found Michael Jackson at 11:00 a.m. not breathing," Chernoff said. He declined to comment on the homicide ruling, saying, "We will be happy to address the coroner's report when it is officially released."
The coroner's office has withheld its autopsy findings, citing a request from police to wait until their investigation is complete.
It is no surprise that such a combination of medications could kill someone, said Dr. David Zvara, anesthesia chairman at the University of North Carolina at Chapel Hill.
"All those drugs act in synergy with each other," Zvara said. Adding propofol on top of the other sedatives could have "tipped the balance" by depressing Jackson's breathing and ultimately stopping his heart, he said.
The 25 milligrams of propofol "is not a whopping amount," said Lee Cantrell, director of the San Diego division of the California Poison Control System. It was the cocktail of the other sedatives, known as benzodiazepines, that "may have been the trigger that pushed him over the edge," Cantrell said.
"This is horrible polypharmacy," he said, referring to the interaction among the various drugs. "No one will treat an insomniac like this."
The affidavit, signed by a judge July 20, said the coroner's office chief medical examiner told police his review of preliminary toxicology results showed "lethal levels of propofol."
Murray didn't tell paramedics or doctors at the UCLA hospital where Jackson was rushed about any drugs he administered other than lorazepam and flumazenil, a "rescue drug" to counteract problems from too much lorazepam, according to the affidavit.
It was only during a subsequent interview with Los Angeles police detectives that Murray gave a fuller accounting of the events leading up to the 911 call, the document said.
The coroner's office said Tuesday it had no comment on the case. A spokeswoman for the Los Angeles District Attorney's said no case had been presented so the office had nothing to comment on.
The line between safe and dangerous doses of propofol is thin, and according to the drug's guidelines a trained professional must always stay bedside. Home use of propofol is virtually unheard of - safe administration requires both a specially trained anesthesiologist and an array of lifesaving equipment. Murray was trained as a heart doctor, not a pain and sedation specialist.
A patient's weight determines the correct dose of a drug like propofol. Jackson was thin and reportedly exhausted from the rigors of rehearsing for a series of summer comeback concerts.
The hazards of getting the dose right is one reason propofol is supposed to be given only by specially trained professionals in a setting where rescue equipment and personnel are immediately available. When the drug is used for surgical anesthesia, doctors typically give one dose to put the patient under and then a lighter dose to maintain anesthesia.
Authorities said in the affidavit that federal Drug Enforcement Administration agents had been unable to find records tying Murray to the acquisition of propofol. By Aug. 11, however, investigators had served a search warrant at a Las Vegas pharmacy and uncovered evidence showing Murray legally purchased the propofol he gave Jackson on the day he died.
Jackson's family members released a statement Monday, saying they have "full confidence" in the legal process and the efforts of investigators. It concludes: "The family looks forward to the day that justice can be served."
Contributing to this report were AP Medical Writer Marilynn Marchione in Milwaukee and Associated Press Writers Alicia Chang in Los Angeles, Michael Gracyzk in Houston and Ken Ritter in Las Vegas.