Michael Jackson was taking prescription drugs as he fought to get into shape for his concert comeback in London in July.
The Jackson family's lawyer Brian Oxman said the pop icon was taking the drugs after suffering injuries during training for his comeback.
Jackson died yesterday morning, Sydney time, after suffering a cardiac arrest, sending shockwaves across the world and tributes pouring in for the tortured music icon revered as the "King of Pop."
The life and times of Michael Jackson
Fans around the globe mourn | Pay tribute to Jackson
VIDEO: Death of an icon | Watch the Thriller video clipThe 50-year-old collapsed at his rented mansion in the exclusive Los Angeles neighbourhood of Holmby Hills. He was rushed to hospital by paramedics before being pronounced dead.
A spokesman for the Los Angeles County coroner said an autopsy was likely to be carried out on Friday, local time. He would not speculate on the cause of death.
Jackson, one of the most influential figures in pop music history, whose four-decade career included the world's highest-selling album, Thriller, had been preparing for a comeback he had dubbed "the final curtain".
He had planned his first series of shows since his child molestation trial in 2005.
Jackson, who fought long-running battles with prescription medication throughout his career, had pushed back the opening dates last month, although organisers insisted the delay was not linked to his health. Randy Phillips, of the concert promoter AEG Live, said at the time: "I would trade my body for his tomorrow. He's in fantastic shape."
But Mr Oxman said he had had concerns about the singer's condition, revealing that Jackson had been taking prescription medication as he prepared for his comeback.
Mr Oxman compared Jackson's fate to the overdose death of the Playboy centrefold model Anna Nicole Smith.
"The people who have surrounded him have been enabling him … if you think that the case of Anna Nicole Smith was an abuse, it was nothing to what we have seen in Michael Jackson's life," he told CNN.
News of Jackson's death triggered an outpouring of grief. Shocked pop music stars, foreign governments and devoted fans paid tribute to him.
His brother Jermaine, the family's official spokesman, later revealed physicians had battled for more than an hour to revive the star after his arrival at the UCLA Medical Centre.
He said Michael Jackson's personal physician, Tohme R. Tohme, was with him at the house and had tried to resuscitate him. "May Allah be with you, Michael, always," Jermaine Jackson said.
A coroner's office helicopter bearing Jackson's body took off llate in the day from the UCLA Medical Centre, where hundreds of journalists and fans had gathered throughout the day.
This pop singer Madonna said: "I can't stop crying over the sad news."
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