Mystery surrounds Michael Jackson's sudden death

One day after Michael Jackson's sudden death, speculation was already turning on Friday to what killed the 50-year-old "King of Pop" just weeks before his long-awaited series of comeback concerts.

Jackson, a former child star who became one of the best-selling pop artists of all time before a descending into a strange and reclusive lifestyle, died on Thursday afternoon at a Los Angeles hospital, where he had been rushed in full cardiac arrest after collapsing at his nearby rental home.

His passing was front page news around the world, airwaves were filled with his greatest hits from "Thriller" to "Billie Jean," social networking sites were bombarded with messages and tributes from fans and musicians continued to pour in.

"It's so sad and shocking," said former Beatle Paul McCartney. "I feel privileged to have hung out and worked with Michael. He was a massively talented boy man with a gentle soul. His music will be remembered forever."

Few details were known about the circumstances surrounding Jackson's death, but the entertainer was reportedly unconscious and not breathing by the time he arrived at UCLA Medical Center, and doctors were unable to revive him.

His body was flown by helicopter from the hospital to the coroner's office late on Thursday.

Brian Oxman, a spokesman for the Jackson family, told CNN on Thursday the family had been concerned about his health and had tried in vain to take care of him for months.

"Michael appeared at rehearsals a couple of times, he was very seriously trying to be able to do those rehearsals," Oxman said of Jackson's preparations for a series of 50 concerts that were scheduled to begin in London in July.

"His use of medications had gotten in the way, his injuries which he had sustained performing, where he had broken a vertebrae and he had broken his leg from a fall on the stage, were getting in the way," Oxman told CNN.

Authorities have scheduled an autopsy for Friday. But they cautioned it could take weeks to determine a cause of death, which will likely have to wait for the return of toxicology tests. Those tests will determine if Jackson had any drugs, alcohol or prescription medications in his system.

Detectives from the Los Angeles Police Department's Robbery Homicide division searched Jackson's home in the upscale Holmby Hills neighborhood of Los Angeles at the behest of Chief William Bratton. But they called the investigation an "every day" event.

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Jackson dominated the charts in the 1980s and is considered one of the most successful entertainers of the past century, with a lifetime sales tally estimated at 750 million records, 13 Grammy Awards and several seminal music videos to his name.