Authorities are seeking to clear up the mystery surrounding Michael Jackson's death, including whether prescription drugs could have been a factor.

An autopsy was planned for Friday, though results weren't likely to be final until toxicology tests could be completed, a process that could take several days and sometimes weeks. However, if a cause can be determined by the autopsy, they will announce the results, said Los Angeles County Coroner Investigator Jerry McKibben.

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Police said they were investigating, standard procedure in high-profile cases.

Brian Oxman, a former attorney of Jackson's and a family friend, said he was concerned about Jackson's use of painkillers and he warned the singer's family about possible abuse.

"I said one day, we're going to have this experience. And when Anna Nicole Smith passed away, I said we cannot have this kind of thing with Michael Jackson," Oxman said Friday on NBC's "Today" show. "The result was, I warned everyone, and lo and behold, here we are. I don't know what caused his death. But I feared this day, and here we are."

The 50-year-old musical superstar died Thursday, just as he was preparing for a series of 50 concerts starting July 13 at London's 02 arena.

He died at UCLA Medical Center after being stricken at his rented home in the posh Los Angeles neighborhood of Holmby Hills. Paramedics tried to resuscitate him for nearly three-quarters of an hour, then rushed him to the hospital, where doctors continued to work on him.

His brother Jermaine said it was believed that Jackson suffered cardiac arrest at his home. Cardiac arrest is an abnormal heart rhythm that stops the heart from pumping blood to the body. It can occur after a heart attack or be caused by other heart problems.

A handful of bleary-eyed fans camped out throughout the night with media outside the Jackson family house in the San Fernando Valley and near his star on the Hollywood Walk of Fame.

Oxman, who said he was speaking on behalf of no one but himself, and other friends made the rounds among the news outlets, adding to the intrigue of Jackson's early demise. Oxman claims Jackson had prescription drugs at his disposal to help with pain suffered when he broke his leg after he fell off a stage and for broken vertebrae in his back.

"When the autopsy comes, all hell's going to break loose, so thank God we're celebrating him now," Liza Minnelli told CBS' "The Early Show" by telephone.

In 2007, Jackson settled a lawsuit filed by a Beverly Hills pharmacy that claimed the singer owed more than $100,000 for prescription drugs over a two-year period.

After Jackson was acquitted on child molestation charges in 2005, Santa Barbara County District Attorney Tom Sneddon argued returning some items belonging to Jackson he labeled "contraband."
 
Sneddon said those included syringes, the drug Demerol and prescriptions for various drugs, mainly antibiotics, that were in different people's names.

Jackson's 1982 album, "Thriller," is the best-selling album of all time, with an estimated 50 million copies sold worldwide.

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