Federal investigators from the Drug Enforcement Administration have searched the offices of Michael Jackson's personal physician, Dr Conrad Murray.
Around 12 DEA agents entered the building in Houston, Texas, with a warrant and accompanied by LAPD detectives. They removed several items.
A representative for the medic, who was with the king of pop when he died, said the morning raid was "a surprise to us and it was a surprise to the attorneys". His lawyer said police were looking for evidence of manslaughter.
The substance Diprivan - a powerful anaesthetic also known as Propofol - was found in the Thriller singer's home after he died.
The investigation into his death is believed to centre around Michael's use of the sedative.
Dr Murray – who was reportedly paid £100,000 a month by the singer - is one of several physicians who have been interviewed in connection with Michael's death. Police say he is not a suspect.
The medic said before the search: "I did not give Michael anything that should have led to his death. I do not know anything more than anyone else about what killed Michael - but it wasn't me."
His lawyer added: "The coroner wants to clear up the cause of death, we share that goal. Based on Dr Murray's minute-by-minute and item-by-item description of Michael Jackson's last days, he should not be a target of criminal charges".