It's a safe combination if done properly; potentially lethal if not.
The official said the type of sedatives Dr. Conrad Murray gave Jackson were benzodiazepines, often used to calm patients before surgery.
Murray told investigators the doses were within normal medical guidelines, said the official, who spoke to The Associated Press on condition of anonymity because the investigation is ongoing.
Even at acceptable levels, benzodiazepines can intensify how the anesthetic Propofol depresses breathing, so strict monitoring and careful dosing is required.
The balance can be tricky, a slip-up disastrous.
Murray, who administered the drugs to Jackson in a room at the pop star's rented mansion, told investigators Jackson stopped breathing the morning of June 25 and he was unable to revive him.
The doctor is the central figure in what police term a manslaughter investigation, though authorities have not classified him a suspect.
In a written statement Thursday, Murray's lawyer, Ed Chernoff, dismissed as "ridiculous" the official's assertion that Murray gave Jackson multiple drugs.
"We are not going to be responding to any accusations from any unnamed sources," Chernoff said.
As investigators build their case, a central issue is what drugs were in Jackson's system when he died.
The official said preliminary toxicology tests detected the Propofol, a drug normally used to render patients unconscious for medical procedures but that Jackson used as a sleep aid.
Further analysis will determine whether other drugs were present and in what quantity, allowing the coroner to conclude whether they contributed to the death.
If a twice-delayed final toxicology report reveals an array of drugs, the answer to the simple question "What killed Michael Jackson?" will become a complex medical and legal dispute, especially if authorities can't prove Murray knew everything Jackson was taking.
That would complicate any prosecution.
If the results show "a bunch of stuff together, unless one of them is sky high it's going to be really tough to give the cause of death to one drug," said Dr. Richard Clark, director of the division of medical toxicology at the University of California, San Diego.
The Los Angeles County coroner uses a form that allows for three causes of death, plus a fourth line for other contributing factors. Given Jackson's long history with prescription and other drugs, Clark said he expects the coroner's office will conclude Jackson died from "multiple drug toxicity."
Anna Nicole Smith and Heath Ledger were found to have died from lethal combinations of drugs.
Craig Harvey, chief investigator at the coroner's office, declined to discuss any findings from the toxicology report or autopsy. He said delays in releasing the findings were due to "investigative issues."
A discovery of several substances in Jackson's system could benefit Murray should he face criminal charges, said Steve Cron, a criminal defence attorney and adjunct professor at Pepperdine University's law school.
"The more drugs in his system, the more deniability (a doctor) has," Cron said, provided Murray could show he didn't know Jackson was on the other drugs.
"But what did the doctor know?" Cron asked. "Did he do his due diligence by trying to find this stuff out, or just look the other way?"
Murray has talked to detectives but has not spoken publicly since Jackson died. Chernoff has said Murray gave Jackson nothing that "should have" killed him and specifically said the physician did not give Jackson the narcotic painkillers Demerol or OxyContin.
The law enforcement official said Demerol was not found in Jackson's home, but the official said a large amount of Propofol and several types of benzodiazepines were in a walk-in closet and the room where Jackson slept his last night.
Benzodiazepines, the "mother's little helper" Mick Jagger sang about, include such widely used antianxiety drugs as Valium and Xanax, as well as Ativan and Versed. The law enforcement official wouldn't name which benzodiazepines Murray told investigators he gave Jackson.
Combining Propofol with benzodiazepines is common in operating rooms. Doing so in a private home is not, and it raises numerous safety issues.
Murray, a cardiologist who was hired as Jackson's personal physician in May, is not an anesthesia specialist, and medical experts question whether he had proper monitoring and lifesaving equipment when he administered the Propofol to Jackson.
Propofol is extremely powerful, and even a small miscalculation in a dose can cause a dangerous drop in breathing and heartbeat. Some benzodiazepines also affect breathing rate, making the combination potentially deadly, according to several anesthesiologists including Dr. John Dombrowski, a board member of the American Society of Anesthesiologists who runs the private Washington Pain Center.
While anesthesiology is rooted in science, some of its top practitioners describe balancing the doses of several drugs as an art. Patients can react differently to the same drugs, especially if they have built up tolerance through long-term use or abuse.
People not properly trained in such nuances are playing within a small margin for error, Dombrowski said.
"We've made it look so simple," Dombrowski said. Nonspecialists may "view it as, 'Well, it should be pretty straightforward,' and they get caught with their pants down."
By Associated Press writers Justin Pritchard and Thomas Watkins