Police wouldn't say whether the killer is one of the victims found inside the house, but did say the family was specifically targeted.
Investigators were forced to come back again Saturday due to the scope of the murder scene and the number of victims.
"There is just too much for them to process in one day," said Cmdr. Lyn Tomioka of the San Francisco Police Department.
Investigators spent the entire day inside the home on Howth Street across from City College. The woman who discovered the bloody scene told police Friday that the victims were her parents, brother and his wife.
The scene was apparently so gruesome that a cause of death for the victims was not easily apparent.
"We shall have to work with the medical examiner's office and rely on them for the cause of death before we can be absolutely sure of what occurred in this residence," said Tomioka.
Todd Sidebottom is a co-worker and friend of the woman who owns the home. When she didn't come to work Friday, he decided to come to the scene because he feared she was among the dead.
"It would be crazy. She was always a happy person, never talked about any troubles at home," he said.
Police say that the killings were not a random act. The people or person in the home were specifically targeted. The reason is still unknown.
Investigators aren't saying how any of the victims were killed but they said the public is not in danger as a result of the crime.
"There is not a murderer on the loose that we are concerned for the public safety of the residents of San Francisco," Tomioka said.