The sheriff's office received a call at about 10:15 a.m. from a relative requesting a welfare check at a home in the 20500 block of Forest Avenue.
When deputies arrived, they found the pair dead inside a guest house detached from the home, sheriff's Sgt. Ray Kelly said.
The man and woman in their 40's appear to have died from gunshot wounds and to have been dead for several days, deputies said. Their names have not been released, and no neighbors that spoke with ABC7 News knew them.
Investigators are trying to determine if the case is a double homicide or murder-suicide, and say it's too early to tell if this was a was a domestic violence incident.
The community is not believed to be in any danger, Kelly said.
#DEVELOPING @ACSOSheriffs will use lasers to create a 3D reconstruction of crime scene where two adults—a man and woman—were found shot to death in a guest house behind a home in #Castrovalley Investigators calling it a double homicide for now, but say could be murder-suicide. pic.twitter.com/SED4yQvNWA— Laura Anthony (@LauraAnthony7) June 27, 2019
#BREAKING @ACSOSheriffs say victims in Castro Valley appear to be in their forties, could be husband and wife, not ruling out murder-suicide. Bodies found in back of home, a guest house, by another family member. May have been there for several days. #abc7now pic.twitter.com/cIlmYbZuz6— Laura Anthony (@LauraAnthony7) June 26, 2019
Stay with ABC7 News for updates on this developing story.