A woman was found dead at the home, and a man and a woman were killed at the business, he said. Two other shooting victims were in critical but stable condition at hospitals.
Milligan said police were looking for a gunman who apparently escaped in a black Camaro that belonged to a victim or a victim's family member.
"Our victims have not been able to give us information as to the identity of the suspect," he said, adding that a motive remained unknown.
Police received two 911 calls -- one from the home and one from the business -- about the attacks. The call from the home was made by a victim, Milligan said.
United States Fire Protection Services is in an industrial area of Downey across from a bottling plant. The family owned business sells professional firefighting gear and equipment such as hoses, valves and extinguishers, according to its website.
Blanca Parker, who works next door, said the owners were very friendly.