Eugene Guevara of El Monte, believed to be in his late 70s, was being sought for investigation of attempted murder in the Friday afternoon shooting at Kaiser Permanente Medical Center, said Lt. Michael Taylor.
It wasn't immediately determined whether Guevara knew and targeted the victim, whose name was withheld.
Authorities released a still photo of the suspect. Taken by a surveillance camera, it showed a man with dark hair and a mustache pushing either a walker or wheelchair down a hospital hallway. It was taken at 3:25 p.m., about the time of the shooting.
The wounded doctor underwent surgery at the hospital for three gunshot wounds. He was in serious but stable condition. He was shot near the third-floor urology department inside the HMO about 17 miles east of Los Angeles.
Lt. Michael Davis said Guevara entered the hospital through an employee entrance sometime after 3 p.m. Friday and opened fire with a small-caliber handgun. Authorities did not immediately determine a motive.
Los Angeles County sheriff's SWAT teams and officers from neighboring police departments searched the hospital room-by-room late Friday, said Deputy Johnny Jones. Authorities now believe the suspect left the hospital after the shooting.
Rudy Lopez was having blood work done when he heard a loudspeaker announcement of an emergency.
"I heard, 'Code red. Code red.' They said it about five or six times," the 68-year-old said.
The intensive care unit and other critical care sections were secured but remained in operation after the shooting, said Los Angeles County fire Capt. Mark Savage. About 15 people were taken to other hospitals.
"I wasn't scared until they opened up the door and I saw the police with the machine guns," said radiology patient Sharon Miller, 41, of San Dimas.
Immediately after the shooting, hundreds of patients, doctors, nurses and others were seen streaming out of the medical complex. Soon after, officers with weapons were seen atop a parking structure, and a sheriff's helicopter carrying a special weapons team landed nearby.
The hospital employs 230 physicians and nearly 2,300 support staff. Of its 269 beds, 190 were in use Friday.
Firefighters were first to arrive, having been called to a small fire that had gone out by the time they arrived, said county fire Inspector Roland Sprewell. When they learned shots had been fired, they notified law enforcement.