The death of pop-singer Michael Jackson has become the focus of a murder investigation, although no charges have  been filed against the main suspect, the entertainer's personal physician, Houston-based Dr. Conrad Murray.  Investigators say Dr. Murray administered a lethal combination of drugs to Jackson in an attempt to overcome the performer's chronic insomnia.

A search warrant affidavit unsealed in Houston shows the mix of sedatives in Michael Jackson's body at the time of his death was a lethal dose.  According to a Los Angeles coroner's investigation, none of the drugs alone would have caused death in the amounts administered, but the combination did.

Since these drugs were administered to the 50-year-old pop singer by his physician, Dr. Conrad Murray, he could be charged with involuntary manslaughter.  The Los Angeles Coroner's office has not issued an official report yet, and the document containing the information about the investigation was released by error Monday in Houston.

Although he has not commented on the report, Dr. Murray has professed his innocence since he first came under suspicion. In a message posted on YouTube last week, he told friends not to worry about him.

"I have done all I could do," he said.  "I told the truth and I have faith the truth will prevail."

Agents of the federal Drug Enforcement Administration raided Dr. Murray's Houston clinic and storage facility last month looking for evidence, but no charges have been filed against him. 

His Houston attorney,  Ed Chertoff, says some of the information in the search warrant affidavit is misleading.  While he says much of the information is factual, he called other parts "police theory."

Anesthesiologists say most of the drugs and doses Dr. Murray administered to Michael Jackson over the several hour period leading up to his death were not dangerous alone, but may have been in combination.  Jackson was suffering from insomnia and was begging the doctor to give him something to make him sleep.

Among the drugs Dr. Murray gave the pop star were Valium and Ativan and Versed. 

But Jackson remained awake and pleaded for a dose of what he called his "milk", a drug called propofol, which is often used in hospitals to induce unconsciousness.

Experts say the propofol dose reportedly given by the doctor would be considered very low, but they say it may have been too much on top of the mix of other drugs already in his system.  Anesthesiologists say it is unusual to use this drug to treat insomnia.

According to the report, Dr. Murray said he had been treating Jackson with propofol on a nightly basis for some time and then stopped out of fear that the pop star was becoming addicted.  He then started using the other drugs to wean Jackson off propofol and they worked well for at least two nights.

In the hours before Jackson died, Dr. Murray says he administered five separate doses of sedatives over a six-hour period and finally gave Jackson the small dose of propofol, which succeeded in putting him to sleep. 

He says he monitored his patient closely for 10 minutes and all was normal when he left to go to the bathroom.  When he returned he found that Jackson was no longer breathing.  He says he tried unsuccessfully to revive him before paramedics arrived and took Jackson to UCLA Medical Center, where he was pronounced dead on the morning of June 25.