Michael Jackson's personal doctor administered a powerful anesthetic to help him sleep, and authorities believe the drug is what killed the pop singer, a law enforcement official told The Associated Press on Monday.

The official, who spoke on condition of anonymity because the investigation is ongoing, said Jackson regularly received propofol to sleep, a practice far outside the drug's intended purpose. On June 25, the day Jackson died, Dr. Conrad Murray gave him the drug sometime after midnight, the official said.

Though toxicology reports are pending, investigators are working under the theory that propofol caused Jackson's heart to stop, the official said.

Murray, 51, has been identified in court papers as the subject of a manslaughter investigation and authorities last week raided his office and a storage unit in Houston. Police say Murray is cooperating and have not labeled him a suspect.

Murray's lawyer, Edward Chernoff, has said the doctor "didn't prescribe or administer anything that should have killed Michael Jackson." When asked Monday about the law enforcement official's statements he said: "We will not be commenting on rumors, innuendo or unnamed sources."

Murray became Jackson's personal physician in May and was to accompany him to London for a series of concerts starting in July. He was staying with Jackson in a rented Los Angeles mansion and, according to Chernoff, found an unconscious Jackson in the pop star's bedroom the morning of June 25. Murray attempted to revive him but could not.

Police searching Jackson's home after his death found propofol and other drugs, an IV line and three tanks of oxygen in Jackson's bedroom, and 15 more oxygen tanks in a security guard's shack.

Propofol can depress breathing and lower heart rates and blood pressure. Because of the risks, propofol is only supposed to be administered in hospitals. Instructions on the drug's package warn that patients must be continuously monitored, and that equipment to maintain breathing, to provide artificial ventilation, and to administer oxygen if needed "must be immediately available."

Jackson had trouble sleeping and the official said he enlisted various doctors to administer propofol, relying on the drug like an alarm clock. He would decide what time he wanted to awaken and at the appointed hour a doctor would stop the intravenous drip that delivered the drug, the official said.