Source: Propofol the Focus of Raid on Dr. Conrad Murray's Houston Office

The Drug Enforcement Agency's Wednesday raid of Michael Jackson's personal physician's Houston office was focused on the powerful anesthetic propofol, a law enforcement official tells FOX News.

A lawyer for Jackson's doctor, Dr. Conrad Murray, says police were searching the clinic for evidence of manslaughter.

The LAPD, which has been in charge of the investigation, served a warrant to Murray's office at Houston's Armstrong Medical Clinic. Murray was present on June 25 at the time of Jackson's death.

In a statement to FOXNews.com, attorney Ed Chernoff said the warrant "authorized law enforcement to search for and seize items, including documents, they believed constitued evidence of the offense of manslaughter."

In addition to 21 documents, Chernoff said police left with a forensic image of a business computer hard drive. None of the documents had been previously requested by investigators, Chernoff said.

The raid began shortly after 11 a.m. EDT Wednesday, when several DEA vehicles accompanied Houston and Los Angeles police departments to the scene.

The sedative propofol, also known as Diprivan, used to put people to sleep before surgery, is not available over the counter or with a prescription, and is rarely used outside of a hospital setting. Paraphenalia related to the use of the drug was found at Jackson's rented Holmby Hills mansion after his death.

FULL COVERAGE: Click here for FOXNews.com's complete coverage of Michael Jackson's death.

The Armstrong Clinic raided Wednesday is owned by Davill Armstrong, who has had previous trouble with the state medical board. In 2006, his license was suspended due to "concerns about Armstrong's judgment, competency, and general medical knowledge," before being reinstated in May. In 2008, he was fined $2,000 for allowing his wife to prescribe medication from the office while his license was suspended and no physician was present.

FOXNews.com reported Tuesday that federal authorities were conducting an investigation in Las Vegas, where Murray also has an office, to follow up on leads indicating that at least one of the drugs that contributed to Jackson's death originated there.

Police questioned Murray twice shortly after Jackson's death, which is being treated as a homicide.

A law enforcement source told FOXNews.com that investigators are using the results of preliminary toxicology reports, which have not yet been released to the public, to track down the sources of the drugs found in high concentrations in Jackson’s system.

Investigators have also been tracking down all drugs recovered from Jackson’s rented Holmby Hills mansion in the days following the singer's death.

The investigative team received assistance from members of local enforcement and spent the day conducting interviews at medical facilities, the source said. The team was expected to return to Los Angeles by Tuesday morning.

FOXNews.com has also learned that the Medical Board of California has started proceedings that could strip Murray of his license to practice medicine.

The Medical Board of California has informed Murray that he is the subject of a malpractice investigation, a law enforcement official told FOXNews.com.

Murray is licensed to practice in California, Texas and Nevada.

The Texas Medical Board and the State of Nevada Board of Medical Examiners are assisting in the California investigation.

Miranda Sevcik, a spokeswoman for Murray’s legal team, said, "We have no information on any investigation by the state medical board."

She says Murray continues to cooperate with police

FOX News's Adam Housley and FOXNews.com's Jana Winter contributed to this report.