The person, who was not authorised to speak publicly and requested anonymity, said Jackson suffered a heart attack, which is a blocking of the arteries that deprives the heart of adequate blood. That can lead to cardiac arrest, an interruption of the normal heartbeat.
Jackson's brother Jermaine said yesterdayy that it was believed the pop singer went into cardiac arrest.
The Los Angeles County coroner's office, which completed its autopsy today, said determining the cause of death will require further tests that will take six to eight weeks.
The possibility of a heart attack could be a key clue as to why Jackson had a cardiologist at his home while he went through vigorous training for an upcoming series of concerts in London: Heart attacks can indicate a long-term problem, such as heart disease. It would not necessarily rule out another factor, such as drug use, however.
Coroner's spokesman Craig Harvey said Jackson was taking some prescription medications, but did not specify what they were.
Police earlier today seized the car of Dr. Conrad Murray, a cardiologist who practices in California, Nevada and Texas, who was with Jackson at the time of his death. Police said they believed the car may contain drugs or other evidence.
When autopsies are performed on cardiac arrest victims, as many as three-fourths show signs of heart disease, such as clogged arteries, said Dr. Douglas Zipes, an Indiana University heart specialist and past president of the American College of Cardiology.
Finding signs of a heart attack would not rule out drugs playing a role.
For example, injections of the powerful painkiller Demerol can depress normal breathing or cause a sudden drop in blood pressure and trigger a heart rhythm problem, said Dr. Lance Becker, a University of Pennsylvania emergency medicine specialist and an American Heart Association spokesman.
If that occurred in someone who already had clogged arteries, it could make the situation much worse, he said.
