Police received reports at about 2:45 p.m. of six or seven shots fired at the medical office near Hoag Hospital in Orange County, Newport Beach police spokeswoman Kathy Lowe said.
Officers responding found a man dead from multiple gunshot wounds to the upper torso in a patient examination room on the building's second floor, Lowe said. They took into custody another man in the room.
The victim was a doctor, Deputy Chief David McGill told the Los Angeles Times. The doctor's name and relationship to the gunman were not immediately clear.
The building includes physician's offices, outpatient services for the hospital and a treatment center for diabetes, but it was not clear which of these the shooting occurred in.
No one else was injured or targeted, Lowe said.
"It does appear to be an isolated incident," she said.
Detectives were interviewing employees and patients in the building.
The building and much of its parking lot were cordoned off with police tape and surrounded with many police vehicles several hours after the shooting.