As the world mourns "King of Pop" Michael Jackson, speculation was turning yesterday to what killed the 50-year-old star just weeks before his long-awaited series of comeback concerts.

In Tanzania, former President Ali Hassan Mwinyi yesterday sent his condolences to the Jackson family as he joined thousands of fans in paying tribute to the pop legend.

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

Few details are known about the circumstances surrounding his death, but the entertainer was reportedly unconscious and not breathing by the time he arrived at Ucla Medical Centre, and doctors were unable to revive him.

Mr Mwinyi, who told the BBC Network Africa Swahili service yesterday he knew Jackson "at a personal level", said he was shocked by the music legend's sudden death.

"He was a young man. I thought he would live longer," the former president told BBC, saying he had once hosted Jackson during his African tour in the 90s.

DJ Dickson Ponela of Magic Radio said: "Michael Jackson proved that blacks are also capable of doing big things." An entertainment editor with a local daily, Mr Henry Mdimu, described Jackson's death as the "falling of a hero".

Brian Oxman, a spokesman for the Jackson family, said 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 scheduled an autopsy for yesterday. But they cautioned it could take weeks to determine the 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. 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.

"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.

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." Kim Khalid, a DJ with East African radio, said the pop star influenced a number of Tanzanian artists who had "copied his singing and dancing".

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.

Reported by Kimani Kim and Bernard Lugongo and agencies