His brother Jermaine said Jackson apparently suffered cardiac arrest, 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.
Jackson was preparing for a monster comeback bid — a series of 50 concerts that was to begin next month in London.
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. People heading to work in New York stopped to pay respects outside Harlem's Apollo Theater, where Jackson performed as a child.
"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.
A producer said Sunday's BET Awards would be dedicated to Jackson because of his influence on music and pop culture. And a screening of Universal Pictures' "Bruno" in Los Angeles on Thursday night cut a scene involving Jackson's sister La Toya.
Jackson's death brought a tragic end to a long, bizarre, sometimes farcical decline from his peak in the 1980s, when he was popular music's premier all-around performer. His 1982 album "Thriller" — which included the blockbuster hits "Beat It," "Billie Jean" and "Thriller" — is the best-selling album of all time worldwide.
Yet after selling more than 61 million albums in the U.S. and having a decade-long attraction open at Disney theme parks, Jackson died reportedly awash in about $400 million in debt, on the cusp of a final comeback after well over a decade of scandal.
The public first knew Jackson as a boy in the late 1960s, when he was the precocious, spinning lead singer of the Jackson 5, the singing group he formed with his four older brothers out of Gary, Ind. Among their No. 1 hits were "I Want You Back," "ABC" and "I'll Be There."
He was perhaps the most exciting performer of his generation, known for his backward-gliding moonwalk, his feverish, crotch-grabbing dance moves and his high-pitched singing, punctuated with squeals and titters. His single sequined glove, tight, military-style jacket and aviator sunglasses were trademarks, as was his ever-changing, surgically altered appearance.
"For Michael to be taken away from us so suddenly at such a young age, I just don't have the words," said Quincy Jones, who produced "Thriller." "He was the consummate entertainer and his contributions and legacy will be felt upon the world forever. I've lost my little brother today, and part of my soul has gone with him."
Jackson ranked alongside Presley and the Beatles as the biggest pop sensations of all time. He united two of music's biggest names when he was briefly married to Presley's daughter, Lisa Marie. Jackson's sudden death immediately evoked comparisons to that of Presley himself, who died at age 42 in 1977.
As years went by, Jackson became an increasingly freakish figure — a middle-aged man-child weirdly out of touch with grown-up life. His skin became lighter, his nose narrower, and he spoke in a breathy, girlish voice. He often wore a germ mask while traveling, kept a pet chimpanzee named Bubbles as one of his closest companions and surrounded himself with children at his Neverland ranch, a storybook playland filled with toys, rides and animals. The tabloids dubbed him "Wacko Jacko."
After the enormous success of "Thriller," Jackson had strong follow-up albums with 1987's "Bad" and 1991's "Dangerous," but his career began to collapse in 1993 after he was accused of molesting a boy who often stayed at his home.
The singer denied any wrongdoing, reached a settlement with the boy's family, reported to be $20 million, and criminal charges were never filed.
Jackson caused a furor in 2002 when he playfully dangled his infant son, Prince Michael II, over a hotel balcony in Berlin while a throng of fans watched from below.
In 2005, he was cleared of charges that he molested a 13-year-old cancer survivor at Neverland in 2003. He had been accused of plying the boy with alcohol and groping him, and of engaging in strange and inappropriate behavior with other children.
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