No one will really know what caused the sudden death of Michael Jackson until an autopsy is performed, but he at least seemed on the way toward being declared fit and ready to tour.

A source close to the production tells E! News the 50-year-old singer passed a medical exam that was required earlier this year in order for promoter AEG Live to obtain the proper insurance from Lloyd's of London for Jackson's 50-show engagement at O2 Arena, which was supposed to kick off in July.

"He passed the physical and everything was a go," the source said. Jackson had been scheduled to undergo another exam prior to his departure.

UCLA Medical Center staff confirmed earlier today that the singer's personal physician was with Jackson at his rented home in the Holmby Hills when paramedics were called sometime after noon.

Sources tell E! that Jackson had been making regular visits to a medical center in Beverly Hills in recent weeks, but it's unclear why.

Meanwhile, AEG has about $40 million riding on what may have caused the King of Pop's untimely demise, according to Billboard.

AEG execs said earlier this year that they were well-insured in case Jackson couldn't go on for unforeseen reasons. Jackson passed a physical "with flying colors," AEG Live CEO Randy Phillips told the trade in May.

But according to insurance-industry experts, a preexisting condition

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