The officials said there is such much evidence to review -- some of it complex medical data -- and that they have sought the help of outside medical experts.
Dr. Conrad Murray, Jackson's personal physician who provided him with the anesthetic propofol that coroner's officials say killed the pop singer, has been identified as a suspect in a manslaughter investigation in search warrants and remains the focus of the probe.
The investigation has resulted in large quantities of information that need to be carefully reviewed not only by LAPD detectives, but also prosecutors and some outside medical advisers before a final decision is made on how to proceed in the case, according to sources familiar with the investigation. They spoke to The Times on condition of anonymity because the investigation is ongoing.
With no suspects in custody, those familiar with the probe compare the Jackson case to the investigation of music producer Phil Spector, which took more than a year before prosecutors decided to file charges.
Murray has denied repeatedly any wrongdoing and insists he gave then-appropriate medical aid when he found Jackson's distressed and not breathing. His attorney has stressed that Murray is fully cooperating with authorities.
The Los Angeles County district attorney's office already has had Murray's girlfriend Nicole Alvarez testify before a grand jury panel, according to sources.
The Los Angeles County coroner's office determined that Jackson died from "acute propofol intoxication" combined with sedatives and labeled his death a homicide.
The complexity of the medical evidence in the case has lead LAPD Robbery Homicide Division detectives and a prosecutor who is virtually embedded with them to seek several medical opinions in addition to the coroner's officials' on Jackson's death, according to two sources.
Murray, 56, told detectives that he administered the anesthetic propofol and other medications to Jackson before the pop star's death June 25, according to search warrants in the investigation. Five bottles of the anesthetic were purchased in Nevada from Applied Pharmacy by Murray, who had it shipped to California, according to search warrants in the investigation.
Some of those bottles were found in Jackson's rented Holmby Hills mansion, according to search-warrant affidavits. Propofol is usually used in medical settings by anesthesia professionals to make patients unconscious for surgery.
The physician told police Jackson was dependent on propofol to sleep and that he was trying to wean the singer off the drug.
Murray last week went back to work at his Houston medical office.