Jackson regularly received the anesthetic propofol to go to sleep. The official, who requested anonymity because the probe is ongoing, told The Associated Press on Monday that Dr. Conrad Murray gave Jackson the drug the last night of his life.
It's not clear if Jackson ever woke up that next day after receiving the drug, reports CBS News correspondent Ben Tracy.
Jackson had a long history of prescription drug use and was under anesthesia for many medical procedures over the years.
Propofol is a powerful anesthetic that should only be used by specially trained medical professionals. Doses of it were found in Jackson's mansion, according to a person with knowledge of the investigation who is not authorized to speak publicly.
Murray was with Jackson when he died June 25 and has been identified in court papers as the subject of a manslaughter investigation.
Murray's lawyer has said the doctor didn't prescribe or administer anything that should have killed Jackson.
But Murray has never denied giving the singer propofol, Tracy reports. The drug is used to sedate people in hospitals during surgery and should never be used outside of a hospital or a medical clinic and if Murray was administering it to Jackson in his home, he would have had to monitor the singer constantly while on the drug.
Police recently shifted into a full-fledged criminal investigation, executing search warrants on Murray's clinic and storage unit and looking ahead to the kind of court case they could build against the physician, legal experts said Friday.
"This is no longer a cause of death investigation," said attorney Mark Geragos, who once represented Jackson. "This is about building a criminal case."
Loyola University law professor Laurie Levenson said she expects to see search warrants served at more places associated with Murray and for investigators to be interviewing his employees.
"I do think they believe there's high suspicion that he may be responsible" for Jackson's death, she said.
Los Angeles police and Drug Enforcement Administration agents executed the warrants Wednesday. The language in the documents said the evidence is being gathered as part of a manslaughter investigation targeting Murray.
Authorities have not publicly termed their investigation criminal and still say Murray is not a suspect.
The items seized included 27 tablets of the weight loss drug phentermine, a tablet of the muscle relaxant clonazepam, business cards, storage receipts, notices from the Internal Revenue Service and computer hard drives.
Authorities also took e-mails from and correspondence addressed to Stacey Howe. Records listed Howe as "administrator" at Murray's Las Vegas business, Global Cardiovascular Associates Inc. Attempts to reach her Friday were unsuccessful.
Murray, 51, was hired as Jackson's personal physician not long before he died. He was in Jackson's rented Los Angeles mansion when the pop star was found unconscious the morning of June 25 and tried unsuccessfully to revive him.
Murray has kept a low profile since Jackson's death. He was interviewed twice by police but has not spoken publicly. Doors to his Las Vegas office were locked Friday with red curtains drawn behind them.
Los Angeles County assistant chief coroner Ed Winter said a final determination on Jackson's cause of death is not expected until the end of next week, when toxicology reports should be finished.
The search warrants granted permission for authorities to seize items "including but not limited to billing records, medication orders, transport receipts, billing receipts, medical records and computerized medical records."
Geragos said that indicates police are following a trail which involves purchases of drugs by Murray and possible shipment of drugs to Jackson's home.
The documents seized from his clinic included a "suspension notice" from Doctor's Hospital in Houston as well as "papers regarding incomplete chart" at the same hospital. Also on the list was an expired medical board certificate.
"I can hear the prosecution opening statement in this case already," Geragos said. "They're going to talk about a doctor who had privileges suspended at a hospital for poor record keeping, has financial difficulties and now he gets this once in a lifetime opportunity to get a large paycheck and be the live-in doctor to Michael Jackson."
Geragos said he was not implying that Jackson committed suicide but that reckless use of a drug or a combination of drugs may have led to death.
Mike Bullard, chief executive officer of Doctor's Hospital, said Murray was on staff and worked in a cardiac lab. He said because Murray lived in Las Vegas he was at the hospital only a few days a month and last was seen there in April.
Bullard refused to confirm whether Murray had been given a suspension notice. But Bullard said that if a suspension notice is not on file with the state of Texas - and the state has no such record - then such a notice would likely be for a minor infraction like a paperwork mistake.
It's unclear whether the seized IRS notices were related to recent financial troubles at Global Cardiovascular. In a 10-month period ending last fall Murray's business was slapped with more than $400,000 in court judgments: $228,000 to Citicorp Vendor Finance Inc. in November 2007, $71,000 to an education loan company in June 2008 and $135,000 to a leasing company last September.
Murray took a leave from his Nevada practice to accompany Jackson to London for a planned series of 50 concerts. He was to be paid a reported $150,000 per month.
Greg Scott, a former U.S. attorney and district attorney in California, said it will take strong evidence to lead prosecutors to make the leap from finding negligence to finding criminal negligence, the basis of a manslaughter charge. Even if a charge is filed, he said prosecutors won't have an easy time at trial.
"Cases against doctors are extremely difficult," he said. "A jury is being asked to second guess the decisions of a trained professional and we usually don't do that. If a doctor tells us to do something, we do it."
He noted that the standard for conviction is beyond a reasonable doubt and, "If the defense finds even one doctor to testify this was proper treatment, that's reasonable doubt."