The Los Angeles County coroner's office said police investigators requested that facts about what killed Jackson remain under a security hold while detectives investigate the events leading up to the pop icon's death.
Neither the coroner's office nor the Police Department would indicate when the results might be made public.
Jackson died June 25 after his personal physician, Dr. Conrad Murray, found him not breathing in bed. Despite efforts to revive him, Jackson never regained consciousness.
Investigators have been trying to determine to what extent medications may have contributed to his death at age 50. Murray told investigators he administered the powerful anesthetic propofol and multiple sedatives to Jackson in the hours before he died, a law enforcement official told The Associated Press. The official spoke on condition of anonymity because the investigation is ongoing.
Murray's attorney, Edward Chernoff, has previously said the doctor did not prescribe anything that "should have" killed Jackson.
The investigation into Murray's background has already seen investigators serve search warrants at his properties in Las Vegas and Houston, where they seized computer hard drives, correspondence and other items. Court documents show investigators consider Jackson's death a possible manslaughter and that, while they are looking at the activities of at least six other doctors, Murray is the target of the investigation.
Keeping Jackson's autopsy sealed gives detectives a better chance to speak candidly with people who may have administered or prescribed drugs to the singer, Loyola University Law School Professor Stan Goldman said.
"If people are trying to get spontaneous statements or something that is not rehearsed or planned, not having the results out might help do that," Goldman said.
He added that if doctors who prescribed certain drugs to Jackson discovered those substances were present in his body when he died, "someone who is not very scrupulous might try to cover their tracks and destroy records."
Los Angeles police spokesman Officer Bruce Borihanh said his department would not answer any questions about the case until the investigation is complete.