Chief Frank Wyant of the Caldwell Police Department said the report of an active shooter at the Portstewart Apartments came in at about 4.20pm on Thursday.
Responding officers found multiple victims at the scene, including a wounded police officer.
The suspect in the case was among the dead, and police said there was no ongoing threat to the public.
The officer's injuries were not life-threatening and he has since been released from hospital.
It was not immediately clear if the people injured and killed in the shooting lived at the complex.
“We are still early in the investigation, and information is still very preliminary,” Caldwell Police Department Lt Joey Hoadley said.