Update 5:58 PM PST In a very brief press conference, an spokesperson for the LAPD's Robbery-Homicide Division offered the additional clarification that Jackson was transported from his home at roughly 1 p.m. PST, nearly 90 minutes before he was pronounced dead at UCLA Medical Center. Robbery Homicide was brought onto the investigation at 4 p.m. PST this afternoon, though the spokesman emphasized the division's involvement was based on the high profile nature of the case and the media involvement, rather than any suspicions in the case.

Update 4:50 PM PST: Speaking on CNN, an LA County Coroner provided more details on the events of this afternoon.  He noted that Jackson was un-responsive and in full cardiac arrest when he was admitted to Ronald Reagan Hospital at UCLA Medical Center. He was pronounced dead  at 2:26 PM PST. The Coroner's office won't confirm the cause of death until after an autopsy is performed which may occur tomrrow.

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Michael Jackson, the King of Pop, has died of cardiac arrest. The 50-year old superstar was in the midst of preparing for a comeback tour, when he collapsed at his home in the Holmby Hills neighborhood of Los Angeles earlier today.

TMZ was the first to report of his death, with LA Times and CBS quickly following with the news. CNN and New York Times have yet to confirm.

The iconic Jackson, who many considered the greatest entertainer of all time,  had been in frail health, but was excited about returning to the concert stage in July when a flight of  50 sold-out dates at O2 Arena in London were slated to begin.  More than 750,000 tickets sold for the shows, which is eerily titled, "The Final Curtain."

AEG's Randy Phillips, who coordinated the concert dates, had told several press outlets when the concert dates were initially announced, stressed that Jackson had passed a physical exam in order to get insurance coverage for the shows.

More as the story develops.