It could be another four to six weeks before the world knows exactly what killed Michael Jackson.

An autopsy was conducted on the pop singer's body on Friday, but officials in Los Angeles said additional tests are needed before a cause of death can be released.

Though the L.A. coroner's office said that Jackson was taking prescription drugs -- with some reports suggesting he was addicted to Oxycontin -- no specific drug use has been confirmed.

Still, there was no trauma on the body and no sign of foul play during the three-hour autopsy Friday, said Craig Harvey, Los Angeles' chief coroner investigator.

Assistant Chief Coroner Ed Winter was with the Jackson family, according to Harvey, who added that release of additional autopsy information will be controlled by a security hold.

The body will be released once the Jackson family picks a final resting place, said Harvey.

Earlier in the day, officials released a chilling 911 call which recorded Jackson's final moments.

A man's voice is heard frantically calling for help in the audio recording, referring to Jackson as a 50-year-old man who urgently needed help because efforts to resuscitate him weren't working.

Transcript of redacted audio recording, released by the L.A Fire Dept:

Caller: I need an ambulance as soon as possible sir.

911 operator: Okay sir, what is your address?

Caller: Los Angeles California. 90077. Carolwood Drive.

911 operator: Tell me exactly what happened.

Caller: Sir, we have a gentleman here that needs help and he's not breathing. We're trying to pump him but he's not breathing.

911 operator: How old is he?

Caller: He's 50 years old.

911 operator: 50... Okay. He's unconscious? He's not breathing?

Caller: Yes, he's not breathing sir.

911 operator: Okay, and he's not conscious either?

Caller: He's not conscious sir.

911 operator: Alright, do you have him on the floor? Where is he at right now?

Caller: He's on the bed.

911 operator: Get him on the floor. I'm going to help you with CPR now. We're on our way there now. I'm going to do what I can to help you over the phone. Did anybody see him ...

Caller: We have his personal doctor here with him sir.

911 operator: You have a doctor there?

Caller: Yes, but he's not responding to anything. He's not responding to CPR, anything

911 operator: Oh, okay. We're on our way there. If your guy is doing CPR instructed by a doctor then he is a higher authority than me. Was anybody with him when this happened?

Caller: No, just the doctor sir. The doctor was the only one here.

911 operator: Okay, so the doctor saw what happened?

Caller: Ah, doctor did you see what happened? (inaudible). Sir, if you please ...

911 operator: We're on our way. I'm just asking these questions ... they're on their way sir.

Caller: Thank you sir. He's pumping, he's pumping his chest but he's not responding to anything.

Police towed away the vehicle, which was left behind Thursday after Jackson died. Police said the vehicle could possibly contain drugs or other evidence relating to the pop star's death.

On Friday afternoon, the Los Angeles Times identified the doctor as a cardiologist, Dr. Conrad C. Murray, who practices in California, Nevada and Texas.

The Associated Press reported that a woman who answered the phone Friday at Murray's Houston clinic confirmed that Murray was Jackson's cardiologist. But she would not confirm the Los Angeles Times report that Murray was performing cardiopulmonary resuscitation on Jackson when paramedics arrived at Jackson's home on Thursday.

Police said the doctor was not under criminal investigation, but coroner's investigators wanted to contact him.

Speculation of drug use

There is growing speculation Jackson was taking powerful prescription painkillers.

Brian Oxman, a former lawyer for Jackson, said Friday he was concerned about Jackson's use of painkillers, and claimed he warned the singer's family about possible abuse.

"I warned everyone, and lo and behold, here we are. I don't know what caused his death. But I feared this day, and here we are," Oxman said.

Oxman claimed Jackson had prescription drugs at his disposal to help with pain suffered when he broke his leg after he fell off a stage and for broken vertebrae in his back.

Celebrity website TMZ claims a close member of Michael Jackson's family told them the singer received a daily injection of Demerol, and the dosage he got Thursday was "too much" and that's what caused his death.

Singing legend Liza Minnelli also chimed in, saying Friday "when the autopsy comes, all hell's going to break loose, so thank God we're celebrating him now."

Around the world, meanwhile, waves of grief.

Heartbroken fans by the millions are paying tribute to the singer who smashed sales records, and defined the music of a generation.

They continue to lay flowers, and say silent prayers, at makeshift memorials outside his home, and at his star on the Hollywood Walk of Fame.

Jackson was planning a massive comeback with a series of 50 shows scheduled for London's O2 Arena -- which would have ended years of seclusion.

The 13-time Grammy-winner, who has sold more than 750 million albums over his career, had not embarked on a major tour since 1997.

Tributes pour in

Giants of the music industry joined fans in paying homage Friday to Michael Jackson:

"I can't stop crying, this is too sudden and shocking. I am unable to imagine this. My heart is hurting. I am in prayer for his kids and the family." -- Singer Diana Ross.

"I am so devastated by this terrible news. From the beginning of my career he was my idol in show business. He was a genius and an incredible artist." -- Canadian Singer Celine Dion

"The incomparable Michael Jackson has made a bigger impact on music than any other artist in the history of music. He was magic. He was what we all strive to be. ... I love you Michael." -- R&B singer Beyonce Knowles.

"He broke barriers, he changed radio formats. With music, he made it possible for people like Oprah Winfrey and Barack Obama to impact the mainstream world. His legacy is unparalleled." -- Hip-hop artist Usher.

Details of death

A fire department official said Jackson was not breathing when paramedics arrived at his rented Bel Air mansion at 12:26 p.m.

Paramedics reportedly tried to resuscitate Jackson for 45 minutes, then rushed him to hospital where doctors took over.

"It is believed he suffered cardiac arrest in his home. However, the cause of his death is unknown until results of the autopsy are known," his brother Jermaine Jackson said.

Cardiac arrest is occurs when an abnormal heart rhythm stops the heart from pumping blood to the body. The condition can occur after a heart attack or can be a result of other heart problems.

Jackson was treated at UCLA Medical Center, just a six-minute drive away. But hospital officials were unable to revive him. Jermaine said doctors pronounced his brother dead at 2:26 p.m.

"A team of doctors, including emergency physicians and cardiologists, attempted to resuscitate him for a period of more than one hour. And they were unsuccessful," he told reporters.

Jermaine asked media to respect his family's privacy "during this tough time, and may Allah be with Michael always."

With files from The Associated Press