Shortly before 5 p.m., officers conducted a welfare check at Mission Gateway Apartments, located at 33215 Mission Blvd., after a concerned neighbor reported not seeing the couple for three days.
Responding officers didn't notice anything suspicious outside of the apartment, but were unable to enter the unit because the apartment's management could not locate the key, according to police.
At about 7:05 p.m., a maintenance worker unlocked the door and located the 90-year-old man and his 59-year-old wife both fatally shot, police reported.
Officers went back to the unit and detectives obtained a search warrant. A handgun was recovered during the search.
The shooting appears to be either a double suicide or murder-suicide and detectives are awaiting the results of autopsy and processing evidence before making a final decision, police reported. The couple is thought to have been dead between one and two days prior to being discovered.
The names of the deceased are being withheld pending notification of family and the results of an autopsy.
Anyone with information regarding the case is asked to call Detective Steve Cesaretti at (510) 471-1365.