Federal and local authorities searched Applied Pharmacy Services on Tuesday morning. The official said agents uncovered evidence that Dr. Conrad Murray legally purchased the anesthetic propofol there.
The official spoke to The Associated Press on condition of anonymity because the investigation is ongoing.
Jackson's death is being investigated as a manslaughter and propofol, which Jackson took as a sleep aid, is a main focus.
Murray's attorney Edward Chernoff had no immediate comment.
Drug Enforcement Administration agents and Los Angeles and Las Vegas police served a sealed search warrant and a notice of inspection around 9:20 a.m. at Applied Pharmacy Services, several miles west of the Las Vegas Strip.
"They are looking at everything that was provided by that pharmacy," including records of controlled substances, DEA spokesman Jose Martinez said.
Dr. Conrad Murray of Las Vegas, Jackson's personal physician who was with the pop star when he died in Los Angeles on June 25, is the focus of what Los Angeles police have called a manslaughter investigation. He has not been charged and police have stopped short of calling him a suspect.
Authorities previously served search warrants at Murray's Las Vegas home and his businesses in Las Vegas and Houston. It was not immediately clear whether Murray had business dealings with Applied Pharmacy.
Investigators are also looking into Jackson's interactions with at least six other doctors, court documents show.
Authorities are trying to determine to what extent medications may have contributed to Jackson's 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 has told The Associated Press. The official spoke on condition of anonymity because the investigation is ongoing.
Michael Flanagan, assistant special agent in charge of the DEA in Las Vegas, said the search warrant was issued "without incident" and the pharmacy staff was cooperating. He said authorities were searching for any and all paper documents and electronic records, but he declined to provide any details.
Agents could be seen wheeling at least four cases from the pharmacy building.
The Los Angeles County coroner's office announced Monday that it has completed its autopsy of Jackson but said it would not release its findings while the police investigation is going on.
Watkins contributed to this report from Los Angeles.
By Ken Ritter and Thomas Watkins