Police said the business and home were owned by the same family and that the gunman likely drove from one location to the other.
Lt. Dean Milligan of the Downey Police Department said police were initially called around 11:15 a.m. to the business, United States Fire Protection, in the 8700 block of Cleta Street. About two minutes later, police received another call about a second shooting at a home about a block and a half west of the business.
"We do believe that for some reason this family was targeted both at their place of business and their residence," Milligan said at a late afternoon press conference. "We do not know why at this point in time, but that being the case, two specific locations this close to each other all involving family members, we do not believe that this act was random."
Milligan said a man and woman were shot to death at the business, and another woman was killed at the residence. Their identities were not released.
In addition to those killed, a woman was wounded at the business and a 13-year-old boy was shot in the arm at the residence. They were transported to area hospitals. One was reported to be in critical but stable condition. The condition of the second injured victim was not known.
A search was under way for at least one suspect, described as an African-American male who was not related to the victims. Police said he took off in a car belonging to one of the victims or a victim's family member. That car was described as a stock, black 2010 Chevrolet Camaro with California license plate 6LEA010. Milligan said the car was missing its front license plate.
A motive was unknown. Surviving victims told officers that they did not recognize the suspect.
Milligan said there were no signs of forced entry at the business or home and that some kind of conversation took place between the gunman and victims.
"It doesn't appear that this person just ran in suddenly and opened fire," Milligan said.
Milligan said footage captured by surveillance cameras will be looked at.
"We were saddened to learn that a shooting has occurred in the city of Downey," said Mayor Mayor Roger C. Brossmer in a statement. "The investigation is still underway and many details are not clear."
"This is a quiet neighborhood and this is scary," said neighbor Sherry Diyorio. "Some guy out here still on the loose that has a gun. It scares me."