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The day after the L.A. County coroner's office completed its autopsy of Michael Jackson, stating that the cause of death had been "deferred" until more tests were done, the pop singer's family requested a second, private investigation of the 50-year-old's body. "We don't know what happened. Was he injected, and with what? All reasonable doubt should be addressed," the Rev. Jesse Jackson told the Associated Press after visiting with the Jackson family.

The question is whether a second investigation will do anything to help clear up the mystery. It's possible, say experts, but not probable.

"In most cases, the second autopsy usually confirms what was done in the first," says Dr. Stephen Cina, deputy chief medical examiner of Broward County in Florida, and one of the pathologists involved in the autopsy of former Playboy pinup girl Anna Nicole Smith in 2007. There may be differences, but Cina warns that any discrepancies could simply reflect the fact that investigators performing the second autopsy would not be working with the same body tissues and fluids that were present in the first autopsy. "The organs have already been dissected once, and with the second autopsy, you are not getting the same pristine blood samples that you got in the first," says Cina.

For example, during autopsy, toxicologists typically take blood samples from the veins that feed directly into the heart from the lower and upper extremities — specifically, from the femoral vein in the thigh or the subclavian vein around the shoulders — which give the most accurate reading of circulating drug levels at any given time. But during a second autopsy, this blood may not be as available, leaving the pathologist no option but to test blood found in body cavities. But that may be problematic, since it likely contains pooled blood and fluids from all over the body, and may not provide a true reading of what substances were circulating in the body at the time of death.

At the very least, however, a separate autopsy conducted by a private forensic pathologist hired by the family could yield the answers the family needs faster than the L.A. County coroner, which will take four to six weeks to issue a full autopsy report.

Why so long? There are two reasons for the lag, says Cina. An office such as the L.A. County coroner's may have a backlog of cases to investigate, and Jackson's case would be added to the back of the list. "The technology itself is usually done within a few days," says Cina. "But sometimes if you're dealing with thousands of cases, things take more time. Every life is important, and every death investigation is important. You don't like to bump people just because someone is more famous."

A private pathologist, with access to a toxicology lab, however, could complete an autopsy within a few days.

A delay may also be caused by the need for more unusual tests. In Anna Nicole Smith's case, for example, pathologists had to send samples of Smith's blood out for additional testing for chloral hydrate, a sleeping medication that is not part of the routine panel of drugs included in an autopsy. If such unusual testing were necessary also in Jackson's case — triggered by traces of unexpected drugs picked up by initial screens — it could push back a final toxicology report by several weeks.

That report, once complete, will reveal what, if any, drugs were in Jackson's body when he died. Pathologists generally start with a urine screen, which can detect five different classes of drugs including opiates (such as the painkillers Demerol, OxyContin and morphine), benzodiazapines, amphetamines and cocaine. If investigators find evidence of drugs in the urine, they screen the blood to determine how much of the substance was present — most drugs remain in the system for several hours, but some take days or, in the case of marijuana, even weeks to clear the body — and whether a drug existed in levels high enough to cause death.

Despite the speculation about the use of Demerol and other prescription painkillers surrounding Jackson's death, the singer's personal physician, Dr. Conrad Murray, said on Monday through his attorney that he never prescribed or administered Demerol or OxyContin to Jackson, and never witnessed Jackson taking either drug.

If Demerol was present in Jackson's system, however, it "shouldn't be too difficult" to test for, says Cina, since the painkiller is part of the routine toxicology screen. If the levels found in the autopsy are high enough, then pathologists may rule that Jackson died of Demerol toxicity. If the drug is present but not at a lethal level, it will be more difficult to determine what role, if any, the drug — which can lower respiration rates — played in Jackson's death. "He could have an elevated but nonlethal level of Demerol, but also four or five other medications at therapeutic but nonlethal levels, that acted in concert to stop his breathing and thereafter stop his heart," says Cina. "We don't know at this point."

In fact, according to Murray's lawyer Ed Chernoff, who spoke on behalf of his client Monday, there is nothing in Jackson's medical history that would have led Murray to believe the singer would go into sudden cardiac arrest or respiratory failure. "Nothing. There was no red flag that was available to Dr. Murray, which would have led him to believe he should have died the way he did," Chernoff said. "It's still a mystery, how he died, to Dr. Murray."

Chernoff told CNN that it was at Murray's suggestion that the Jackson family requested a second autopsy. "He needed to know as well, as his physician, what caused Michael Jackson to stop breathing," Chernoff said.

• Alice Park
• The Jackson family has requested a second, private autopsy. Is it necessary? And what might it show?