EXCLUSIVE: The investigative team heading the probe into Michael Jackson’s death conducted interviews with physicians and personnel at medical facilities in Las Vegas on Monday, a law enforcement source close to the investigation told FOXNews.com.

Lead investigators from the Los Angeles Police Department’s Robbery and Homicide Division, along with several field agents from the Los Angeles Drug Enforcement Administration office, took a day trip to Las Vegas to follow up on leads indicating that at least one drug believed to have contributed to Jackson’s sudden death on June 25 originated in Las Vegas.

Sources could not tell FOXNews.com what drug was the focus of the probe, who was questioned or which medical practices were visited, but Jackson’s personal physician Conrad Murray runs a cardiology practice in Las Vegas.

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

Murray is believed to currently be staying at his family's home in Texas. It is unclear if he was part of Monday's investigation, but it was reported that the doctor canceled his third interview with LAPD last week. A spokeswoman for Murray's legal team has denied those reports.

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

The 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 death of the King of Pop.

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.