Prosecutors are prepared to seek an indictment of Michael Jackson's doctor on a charge of involuntary manslaughter in the pop star's death, The Associated Press learned on Friday. A law enforcement source who spoke on condition of anonymity because the investigation remains open said that Dr. Conrad Murray would be prosecuted on a theory of gross negligence alleging that his treatment of Jackson was an extreme departure from the standard of care normally followed by physicians. The coroner has ruled Jackson's death at age 50 a homicide with his death caused by acute intoxication of the powerful anesthetic propofol with other sedatives a contributing factor. Propofol depresses breathing and the heart rate and lowers blood pressure so it's supposed to be administered by an anesthesia professional in a medical setting. The coroner found the propofol was administered to Jackson without any medical need and that recommended resuscitation equipment was missing. The singer was found to be in relatively good health for a man his age and no illegal drugs were detected in his system. Murray, a cardiologist, was with the star at his rented Los Angeles mansion in June and tried to revive him when he was found unconscious. Miranda Sevcik, a spokeswoman in Houston for Murray and his lawyer, Edward Chernoff, said Friday the doctor had no comment and reiterated that Murray neither prescribed nor administered anything that should have killed Michael Jackson. Jackson died while under Murray's care as the singer prepared for an ambitious concert schedule. The district attorney's office is waiting for Los Angeles police to turn over the case before presenting it to a grand jury, the law enforcement source said. A spokeswoman for the district attorney's office, however, denied that any decisions have been made. "We have been working closely with the Los Angeles police during the pendency of this investigation," spokeswoman Sandi Gibbons said. "There is no case before us at present and no final decision has been made." To prove a charge of involuntary manslaughter, authorities must show there was a reckless action that created a risk of death or great bodily injury. If a doctor is aware of the risk, there might also be an issue of whether the patient knows that risk and decided to take it. A large number of witnesses have been interviewed by police, including those who were present during Jackson's last days and those who worked with him in preparation for his comeback concert, "This Is It." Authorities have also lined up medical expert witnesses who will testify about the normal standard of care in a situation such as Jackson's and will give opinions on why Murray's actions constituted gross negligence, the source said. The police investigation was substantially completed by the end of December, the source said. Murray's professional history is expected to be explored during a trial with an emphasis on whether he had the required expertise in administering propofol. The timing of an indictment will be dictated by two factors - how long it takes for the district attorney's office to conduct an internal review of the evidence and when the grand jury will be available to hear the case. The person said it was thought that it would be more efficient to go to a grand jury than to charge Murray and proceed by way of a preliminary hearing. A presentation to the grand jury where witnesses testify behind closed doors could take three to five days. Murray has offices in Las Vegas and Houston. He was hired by Jackson not long before the pop star's death to travel with him on the tour that was to begin in London.