LOS ANGELES — The final act of Michael Jackson's life came into clearer focus Friday, a picture of a fallen superstar working out with TV's "Incredible Hulk" and under the care of his own private cardiologist as he tried to get his 50-year-old body in shape for a grueling bid to reclaim his glory.

While the exact circumstances of his death remained unclear, early clues suggested he may simply have pushed his heart too far.

Police said they had towed the doctor's BMW from Jackson's home because it may include medication or other evidence, and a source familiar with the situation told The Associated Press that a heart attack appeared to have caused the cardiac arrest that led to the pop icon's sudden death.

As grief for the King of Pop poured out from the icons of music to heartbroken fans, and the world came to grips with losing one of the most luminous celebrities of all time, an autopsy showed no sign of trauma or foul play to Jackson, who died Thursday at UCLA Medical Center after paramedics not could not revive him.

The AP source who said Jackson apparently suffered a heart attack was not authorized to speak publicly and requested anonymity. Jackson's brother Jermaine had said the pop singer apparently went into cardiac arrest — which often, but not always, happens because of a heart attack.

Authorities said they spoke with the doctor briefly Thursday and Friday and expected to meet with him again soon. Police stressed that the doctor, identified by the Los Angeles Times as cardiologist Conrad Murray, was not a criminal suspect.

"We do not consider him to be uncooperative at this time," Beck said. "We think that he will assist us in coming to the truth of the facts in this case."

Craig Harvey, a spokesman for the Los Angeles County coroner, said there were no signs of foul play in the autopsy and further tests would be needed to determine cause of death. He said Jackson was taking some unspecified prescription medication but gave few other details.

Meanwhile, a 911 call released by fire officials shed light on the desperate effort at the mansion to save Jackson's life before paramedics arrived Thursday afternoon. Jackson died later at UCLA Medical Center.

In the recording, an unidentified caller pleads with authorities to send help, offering no clues about why Jackson was stricken. He tells a dispatcher that Jackson's doctor is performing CPR.

"He's pumping his chest," the caller says, "but he's not responding to anything."

Asked by the dispatcher whether anyone saw what happened, the caller answers: "No, just the doctor, sir. The doctor has been the only one there."