Los Angeles police and federal Drug Enforcement Administration agents entered Dr. Conrad Murray's home in a gated community while others across town searched Murray's medical offices, Global Cardiovascular Associates Inc.
Michael Flanagan, assistant special agent in charge of the Las Vegas DEA office, said the warrants were sealed and he couldn't say what documents agents were seeking. He said he expected the search would last five hours.
It was the second time in less than a week that investigators targeted Murray's property. Last week in Houston authorities searched his clinic and storage. Court records show they were seeking evidence of whether the doctor committed manslaughter.
With toxicology reports pending, investigators are working under the theory that Murray gave Jackson the powerful anesthetic propofol to help him go to sleep, and the drug caused his heart to stop, a law enforcement official told The Associated Press.
Murray told investigators he administered propofol sometime in the early morning of June 25, the day Jackson died, according to the official, who spoke on condition of anonymity because the investigation is ongoing.
The official told AP that Murray left the bedroom where Jackson was sedated and returned to find the star unresponsive.
Police have said Murray is cooperating and have not labeled him a suspect.
When authorities arrived Tuesday, Flanagan said the doctor was at his Las Vegas home located on a cul de sac near the 18th hole of a golf course. Murray kept out of sight of neighbors who milled about outside the house and the reporters the residents had invited into the private community.
Flanagan said staff members at Murray's medical office were cooperative.
Murray's lawyer in Houston, Edward Chernoff, has said the doctor "didn't prescribe or administer anything that should have killed Michael Jackson." Late Monday, his office posted a statement online that said, "things tend to shake out when all the facts are made known, and I'm sure that will happen here as well."
On Tuesday, Chernoff's spokeswoman said the lawyer had no immediate comment on the searches.
Jackson is believed to have been using propofol for about two years and investigators are trying to determine how many other doctors administered it. Murray told investigators he had given Jackson the drug several times before, the official said.
Jackson relied on the drug like an alarm clock, the official said, explaining a doctor would administer it when Jackson went to sleep then stop the IV drip when the singer wanted to wake up.
Using propofol to sleep exceeds the drug's intended purpose. The drug can depress breathing and lower heart rates and blood pressure.
Home use of propofol is virtually unheard of. Because of its risks, propofol is supposed to be administered only in medical settings by trained personnel.
Guidelines issued by the American Society of Anesthesiologists say "during the administration of propofol, patients should be monitored without interruption."
Murray, 51, 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 the Los Angeles mansion and, according to Chernoff, "happened to find" an unconscious Jackson in the pop star's bedroom the morning of June 25. Murray tried to revive him by compressing his chest with one hand while supporting Jackson's back with the other.
The law enforcement official also provided a glimpse into how the pop star was living in the weeks before he died, describing the room in which Jackson slept in his rented Beverly Hills mansion as outfitted with oxygen tanks and an IV drip.
Police found drugs in the home, including a stash of propofol and other liquid medications such as sedatives big enough to fill two gym bags. The drugs were hidden in a closet.
An IV line and three tanks of oxygen were in the room where Jackson slept and 15 more oxygen tanks were in a security guard's shack, the official said.
