The gunman reportedly barricaded himself inside Family Dental Care office for an hour, according to Simi Valley police spokesman John Stires. A hostage negotiator convinced the man to surrender. The suspect remains in police custody.
Three people were taken to a local hospital in critical condition; another person suffered minor injuries but was not transported to a hospital. That person had been grazed by a bullet.
Police said the shooting was at a dentist and chiropractor office, with Christian Auto Repair Service behind the office, on the 400 block of Los Angeles Ave. At least 12 people were evacuated from the dentist's office and the chiropractic office next door.
A motive for the shooting has not been determined. A Ventura County Star newspaper report stated that a dental office worker said the gunman was married to the slain woman.
The suspected shooter was a man with a shaved head, wearing shorts and no shirt.
The Associated Press contributed to this report.