The bacterium was discovered in a water source at the Holmby Hills estate.
But L.A.'s county health director says the county hasn't ruled out other bacteria or illnesses that could have caused the outbreak.
Two hundred people attended an event at the mansion last month as part of a conference.
In all, some 170 people who attended the event got sick. Several were diagnosed with Pontiac Fever, a mild form of Legionnaires' disease. Its symptoms are similar to pneumonia, with high fever, chills and a cough. Experts say the condition usually clears up on its own within two to five days.