Los Angeles coroners began an autopsy on Friday to determine the cause of U.S. pop star Michael Jackson's death as rumors spread that he might die from a drug overdose as well as heart attack.

US pop star Michael Jackson gestures during a news conference at the O2 Arena in London in this March 5, 2009. [Xinhua]

Jackson, who was 50, was pronounced dead Thursday afternoon at the Medical Center of the University of California in Los Angeles (UCLA). He was in full cardiac arrest when paramedics rushed him to the hospital.

As fans across the world mourned Jackson's sudden death, his associates have been telling news outlets he had a long history of abusing prescription painkillers.

The exact cause of his death may not be determined for six to eight weeks, depending on what type of tests are ordered, according to Craig Harvey of the Los Angeles County Department of Coroner.

The results of toxicological tests usually take that long to return in L.A. County, officials said.

Jackson's brother, Jermaine, told reporters at the hospital that his brother, "the legendary King of Pop," died at 2:26 p.m.

"It is believed he suffered cardiac arrest in his home, however, the cause of his death is unknown until the results of the autopsy are known," he said. His personal physician, who was with him at the time, attempted to resuscitate my brother, as did the paramedics who transported him to Ronald Reagan UCLA Medical Center.

Detectives from the Los Angeles Police Department's Robbery-Homicide division were sent to Jackson's rented home to investigate the death. Lt. Gregg Strenk told reporters at the home that it is routine for police to investigate any death, but the elite Robbery/Homicide detail was sent solely based on the high-profile nature of Jackson's death.

Police were reportedly looking for Jackson's live-in doctor, who had allegedly been giving the performer shots of the strong painkiller Demerol.

Pop star Michael Jackson, in this Santa Maria, California June 13, 2005. [Xinhua]

Police towed the car of a doctor from Michael Jackson's home Friday and said it could contain drugs or other evidence offering clues in the pop star's death.

Meantime, probate questions are already being raised, including who will gain custody of his three children, and what will become of his assets, including his 50 percent ownership of the Beatles songbook.

Jackson had two children by nurse Debbie Rowe -- Prince, 12, and Paris, 11. He said his third child, Prince Michael II, nicknamed Blanket, was conceived using a surrogate mother he never met.

Jackson's former attorney, Brian Oxman, said the children are currently with Jackson's grandmother.

The pop star had been rehearsing for an arduous series of 50 concerts in London, and producer AEG may put as much as 30 million dollars on the concert tour, according to broadcast reports.

The concerts were seen both as a comeback and a way to relieve his financial problems. In 2008, he had to sell his Neverland Ranch outside Santa Barbara, California. It was reported that the London concerts could have earned him as much as 400 million dollars.

Jackson, who became known as the "King of Pop" with the success of albums such as "Thriller," "Bad" and "Dangerous," first gained national exposure with his brothers -- Tito, Jermaine, Jackie and Marlon -- as part of the Jackson 5. The group opened for artists such as Gladys Knight and the Pips and James Brown.

Over his career, Jackson won 13 Grammys, and at this weekend's BET Awards, many of the winners are expected to express their gratitude to him.