Los Angeles police are looking for Jackson's doctor, who they hope can shed light on how the pop singer died. Cops towed a silver BMW from outside Jackson's Los Angeles home, which police confirmed belonged to Jackson's "personal physician" and which they believed contains evidence crucial to the investigation. The doctor's name has not been officially released by authorities. Law enforcement sources, however, confirmed to ABC News, that the car towed from Jackson's home is registered to one Susan Mary Rush. Rush is the sister of Dr. Conrad Robert Murray, who was the doctor with Jackson Thursday. Police interviewed Murray Thursday night, but have not been able to reach him since then. "The car was impounded," said Amanda Betat, a spokeswoman for the Los Angeles Police Department. "One reason it was impounded was because it may contain medication or evidence that could assist the coroner in determining the cause of death." The British Newspaper the Sun has reported that Jackson was given a lethal dose of the painkiller Demerol before his death.