The shooting by a 31-year-old man with a semi-automatic rifle started about 3:30 p.m. Saturday at the Walmart distribution center south of Red Bluff, a city of about 14,000 people about 131 miles (210 kilometers) north of Sacramento, California.
Red Bluff Police officers shot and killed the suspect, KHSL-TV reported.
Officials did not identify the shooter, but Johnston said he has a history with the Red Bluff workplace.
The shooting victims were taken to St. Elizabeth Community Hospital in Red Bluff, spokeswoman Allison Hendrickson told The Associated Press.
Scott Thammakhanty, an employee at the facilitys receiving center, told the Redding Record-Searchlight that he heard the shooter fire from the rifle.