The incident occurred in Santa Monica College, California, that ended Friday with the gunman clad in tactical gear, dead in a college library and a "person of interest" in custody, authorities said.
"We are not yet convinced whether the suspect who was killed operated alone", Santa Monica police chief Jacqueline Seabrooks told reporters.
The comments came as police and firefighters searched the sprawling campus of Santa Monica College for other possible victims or shooters. By early evening, authorities declared the campus clear.
The gunman has not been identified by authorities, Seabrooks said.