Doctors conducted an autopsy on the body of Michael Jackson on Friday but could not immediately determine what killed the "King of Pop," amid reports he had been injected with a narcotic painkiller shortly before collapsing.

Jackson was in full cardiac arrest when paramedics arrived at his rented mansion in the Holmby Hills neighborhood of Los Angeles on Thursday afternoon, with his personal physician trying desperately to revive him.

The 50-year-old pop superstar was rushed to nearby UCLA Medical Center, where he was pronounced dead without regaining consciousness.

"The cause of death (determination) has been deferred, which means that the medical examiner has ordered additional testing such as toxicology and other studies," Los Angeles County Coroner's spokesman Craig Harvey said. "Those tests we anticipate will take an additional four to six weeks."

Speaking to a throng of reporters outside the coroner's office, Harvey said, "There was no indication of any external trauma or indication of foul play to the body of Jackson."

Police said they were seeking to question Jackson's personal physician, identified by news media as Houston-based cardiologist Dr. Conrad Murray.

Jackson's body will be released to family members after they choose a mortuary for funeral arrangements, Harvey said. There was no immediate word on when he would be laid to rest.

Celebrity website TMZ.com, citing an interview with an unidentified "close member" of the Jackson family, reported the entertainer was injected with Demerol about half an hour before he went into cardiac arrest.

Narcotic painkiller

TMZ, citing family members, said Jackson received a daily shot of Demerol, a narcotic painkiller, and that the family believed his death was caused by an overdose of the drug.

Detectives searched Jackson's home and impounded Murray's Mercedes from the driveway, saying it might contain evidence.

An unidentified man called a 911 emergency phone line from the mansion at 12:21 p.m. local time, saying Jackson was unconscious and not breathing.

In excerpts from the call released by authorities, the caller said the physician was the only other person present and was frantically performing cardiopulmonary resuscitation on the unconscious Jackson without results.

"He's pumping, he's pumping his chest but he's not responding to anything, sir, please," the man said.

A senior law enforcement official told ABC News that Jackson was "heavily addicted" to the painkiller Oxycontin and was injected daily with that medication, along with Demerol.

Lawyer Brian Oxman, a Jackson family spokesman, told CBS' "The Early Show" he had been concerned about the prescription drugs Jackson took due to injuries suffered while performing.

"I do not want to point fingers at anyone because I want to hear what the toxicology report says and the coroner says but the plain fact of the matter is that Michael Jackson had prescription drugs at his disposal at all times," Oxman said.

Fans and fellow pop stars revived memories of Jackson's musical genius, tarnished over the past decade by accusations of child molestation and eccentric behavior.

US president Barack Obama called Jackson a "spectacular performer" but said he believed aspects of his life were "sad and tragic," the White House said.

Fans pay tribute

Jackson's death was front-page news around the world as airwaves filled with his greatest hits from "Thriller" to "Billie Jean" and social networking sites were bombarded with messages and tributes.

"My heart, my mind are broken," actress Elizabeth Taylor, long a close friend of Jackson, said in a statement.

"He will be in my heart forever but it's not enough," Taylor said. "My life feels so empty. I don't think anyone knew how much we loved each other."

On Hollywood Boulevard, police put up barricades to con