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.

Few details were known early on Friday about the circumstances surrounding his 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 that 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 that 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.

The London concert swing was billed as a comeback for Jackson, who dominated the pop charts during the 1980s with such hits as Thriller and Billie Jean and was credited with turning music videos into a costly and cinematic art form.

750 MILLION RECORDS SOLD

He is considered one of the most successful entertainers of the past century, with a lifetime sales tally estimated at 750 million records and 13 Grammy Awards.

Michael was and will remain one of the greatest entertainers that ever lived, said Motown Records founder Berry Gordy, Jackson's first label boss. He was exceptional, artistic and original. He gave the world his heart and soul through his music.

But Jackson's reputation as a singer and dancer was overshadowed in recent years by his increasingly abnormal appearance, and bizarre lifestyle, which included his friendship with a chimp and a preference for the company of children.

He named his estate in the central California foothills Neverland Valley Ranch, in tribute to the J.M. Barrie's Peter Pan stories, and filled it with amusement park rides and a petting zoo.

Jackson was twice accused of molesting young boys and was charged in 2003 with child sexual abuse. He became even more reclusive following his 2005 acquittal and vowed that he would never again live at Neverland.

Facing a battered reputation and mountain of debts that the Wall Street Journal reported ran to $500 million, Jackson had spent the last two months rehearsing for the London concerts, including Wednesday night at the huge Staples Center arena, home to the Los Angeles Lakers basketball team.

Despite reports of Jackson's ill-health, the promoters of the London shows, AEG Live, said in March that Jackson had passed a 4-1/2 hour physical examination with independent doctors.

I can't stop crying over the sad news, pop star Madonna said in a statement. I have always admired Michael Jackson. The world has lost one of the greats but his music will live on forever.

Jackson was born on August 29, 1958, in Gary, Indiana, the seventh of nine children and first performed with his brothers as a member of the Jackson 5.

His 1982 album Thriller yielded seven top-10 singles. The album sold 21 million copies in the United States and at least 27 million internationally.

The following year, he unveiled his signature moonwalk dance move, gliding across the stage and setting off an instant trend, while performing Billie Jean during an NBC special.

In 1994, Jackson married Elvis Presley's only child, Lisa Marie, but the marriage ended in divorce in 1996.

I'm so very sad and confused with every emotion possible. ... This is such a massive loss on so many levels, words fail me, Presley said in statement.

Jackson married Debbie Rowe the same year and had two children, before splitting in 1999, and he later had another child with an unidentified surrogate mother.

He is survived by three children named Prince Michael I, Paris Michael and Prince Michael II, known for his brief public appearance when his father held him over the railing of a hotel balcony, causing widespread criticism.

(Editing by Dean Goodman and Anthony Boadle)

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